タイトル通りの内容です。
Rails8 を試してみてたところactive adminが必要になりactiveadminのリポジトリをみたらv4がベータリリースされてたので試してみました。(tailwindcss v4は既に導入済み)

執筆時点(2025/03/24)ではまだrails8, tailwindcss v4は未対応でベータ版をgemfileに追加しても正しく動作してくれないのでその修正方法です。

Ruby on Rails 8 (執筆時点 v8.0.2)
Tailwindcss v4 (執筆時点 v4.2.1)
Activeadmin (執筆時点 4.0.0.beta15)

通常通りgemfileに gem "activeadmin", "4.0.0.beta15" を追加した後、rails g active_admin:install をします。
その後いくつかのファイルを修正します。

変更点は以下です。

activeadmin gem内

app/views/app/views/active_admin/_html_head.html.erb

- <%= stylesheet_link_tag "active_admin" %>
+ <%= stylesheet_link_tag :active_admin %>

lib/generators/active_admin/assets/templates/active_admin.css

- @tailwind base;
- @tailwind components;
- @tailwind utilities;
+ @import "tailwindcss";

+ /* @config "../../../config/tailwind-+ active_admin.config.js"; */
+ @config "path/to/tailwind-active_admin.config.js";

lib/generators/active_admin/assets/templates/tailwind.config.js

- import { execSync } from 'child_process';
- import activeAdminPlugin from '@activeadmin/activeadmin/plugin';
-
- const activeAdminPath = execSync('bundle show activeadmin', { encoding: 'utf-8' }).trim();
-
- export default {
-  content: [
-    `${activeAdminPath}/vendor/javascript/flowbite.js`,
-    `${activeAdminPath}/plugin.js`,
+ execSync = require('child_process').execSync;
+ const activeAdminPath = execSync('bundle show activeadmin', { encoding: 'utf-8' }).trim();
+ const activeAdminPlugin = require(`${activeAdminPath}/plugin.js`).default;
+
+ module.exports = {
+  content: [
+    `${activeAdminPath}/vendor/javascript/flowbite.js`,
+    `${activeAdminPath}/plugin.js`,

lib/generators/active_admin/install/install_generator.rb

+      # def create_migrations
+      # ...
+      # end
+      
+      # create_migrationsブロックの下に追加
+      def copy_stylesheets
+        template "active_admin.css", "app/assets/stylesheets/active_admin.css"
+      end

plugin.js (長いので折りたたんでいます。差分は割愛)

plugin.js

import plugin from 'tailwindcss/plugin';
import defaultTheme from 'tailwindcss/defaultTheme';
import colors from 'tailwindcss/colors';
const { spacing, borderWidth, borderRadius } = defaultTheme;

// https://github.com/tailwindlabs/tailwindcss/discussions/9336
// https://github.com/tailwindlabs/tailwindcss/discussions/2049
// https://github.com/tailwindlabs/tailwindcss/discussions/2049#discussioncomment-45546

const svgToTinyDataUri = (() => {
  // Source: https://github.com/tigt/mini-svg-data-uri
  const reWhitespace = /\s+/g,
    reUrlHexPairs = /%[\dA-F]{2}/g,
    hexDecode = { '%20': ' ', '%3D': '=', '%3A': ':', '%2F': '/' },
    specialHexDecode = match => hexDecode[match] || match.toLowerCase(),
    svgToTinyDataUri = svg => {
      svg = String(svg);
      if (svg.charCodeAt(0) === 0xfeff) svg = svg.slice(1);
      svg = svg
        .trim()
        .replace(reWhitespace, ' ')
        .replaceAll('"', '\'');
      svg = encodeURIComponent(svg);
      svg = svg.replace(reUrlHexPairs, specialHexDecode);
      return 'data:image/svg+xml,' + svg;
    };
  svgToTinyDataUri.toSrcset = svg => svgToTinyDataUri(svg).replace(/ /g, '%20');
  return svgToTinyDataUri;
})();

export default plugin(
  function({ addBase, addComponents, theme }) {
    addBase({
      [[
        "[type='text']",
        "[type='email']",
        "[type='url']",
        "[type='password']",
        "[type='number']",
        "[type='date']",
        "[type='datetime-local']",
        "[type='month']",
        "[type='search']",
        "[type='tel']",
        "[type='time']",
        "[type='week']",
        'textarea',
        'select',
      ]]: {
        appearance: 'none',
        'background-color': '#fff',
        'border-color': theme('colors.gray.500', colors.gray[500]),
        'border-width': borderWidth['DEFAULT'],
        'border-radius': borderRadius.none,
        'padding-top': spacing[2],
        'padding-right': spacing[3],
        'padding-bottom': spacing[2],
        'padding-left': spacing[3],
        '--tw-shadow': '0 0 #0000',
        '&:focus': {
          outline: '2px solid transparent',
          'outline-offset': '2px',
          '--tw-ring-inset': 'var(--tw-empty,/*!*/ /*!*/)',
          '--tw-ring-offset-width': '0px',
          '--tw-ring-offset-color': '#fff',
          '--tw-ring-color': theme(
            'colors.blue.600',
            colors.blue[600]
          ),
          '--tw-ring-offset-shadow': `var(--tw-ring-inset) 0 0 0 var(--tw-ring-offset-width) var(--tw-ring-offset-color)`,
          '--tw-ring-shadow': `var(--tw-ring-inset) 0 0 0 calc(1px + var(--tw-ring-offset-width)) var(--tw-ring-color)`,
          'box-shadow': `var(--tw-ring-offset-shadow), var(--tw-ring-shadow), var(--tw-shadow)`,
          'border-color': theme('colors.blue.600', colors.blue[600]),
        },
      },
      [['input::placeholder', 'textarea::placeholder']]: {
        color: theme('colors.gray.500', colors.gray[500]),
        opacity: '1',
      },
      ['::-webkit-datetime-edit']: {
        display: 'inline-flex',
      },
      [[
        '::-webkit-datetime-edit',
        '::-webkit-datetime-edit-year-field',
        '::-webkit-datetime-edit-month-field',
        '::-webkit-datetime-edit-day-field',
        '::-webkit-datetime-edit-hour-field',
        '::-webkit-datetime-edit-minute-field',
        '::-webkit-datetime-edit-second-field',
        '::-webkit-datetime-edit-millisecond-field',
        '::-webkit-datetime-edit-meridiem-field',
      ]]: {
        'padding-bottom': '0',
        'padding-top': '0',
      },
      ['::-webkit-date-and-time-value']: {
        'min-height': '1.5em',
        'text-align': 'inherit',
      },
      ['select']: {
        'background-image': `url("${svgToTinyDataUri(
          `<svg aria-hidden="true" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 10 6">
            <path stroke="${theme('colors.gray.500', colors.gray[500])}" stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="m1 1 4 4 4-4"/>
          </svg>`
        )}")`,
        'background-position': `right ${spacing[3]} center`,
        'background-repeat': `no-repeat`,
        'background-size': `0.75em 0.75em`,
        'padding-inline-end': spacing[8],
        'print-color-adjust': `exact`,
      },
      [':is(:where([dir=rtl]) select)']: {
        'background-position': `left ${spacing[3]} center`,
      },
      ['[multiple]']: {
        'background-image': 'initial',
        'background-position': 'initial',
        'background-repeat': 'unset',
        'background-size': 'initial',
        'padding-inline-end': spacing[3],
        'print-color-adjust': 'unset',
      },
      [[`[type='checkbox']`, `[type='radio']`]]: {
        appearance: 'none',
        padding: '0',
        'print-color-adjust': 'exact',
        display: 'inline-block',
        'vertical-align': 'middle',
        'background-origin': 'border-box',
        'user-select': 'none',
        'flex-shrink': '0',
        height: spacing[4],
        width: spacing[4],
        color: theme('colors.blue.600', colors.blue[600]),
        'background-color': '#fff',
        'border-color': theme('colors.gray.500', colors.gray[500]),
        'border-width': borderWidth['DEFAULT'],
        '--tw-shadow': '0 0 #0000',
      },
      [`[type='checkbox']`]: {
        'border-radius': borderRadius['none'],
      },
      [`[type='radio']`]: {
        'border-radius': '100%',
      },
      [[`[type='checkbox']:focus`, `[type='radio']:focus`]]: {
        outline: '2px solid transparent',
        'outline-offset': '2px',
        '--tw-ring-inset': 'var(--tw-empty,/*!*/ /*!*/)',
        '--tw-ring-offset-width': '2px',
        '--tw-ring-offset-color': '#fff',
        '--tw-ring-color': theme('colors.blue.600', colors.blue[600]),
        '--tw-ring-offset-shadow': `var(--tw-ring-inset) 0 0 0 var(--tw-ring-offset-width) var(--tw-ring-offset-color)`,
        '--tw-ring-shadow': `var(--tw-ring-inset) 0 0 0 calc(2px + var(--tw-ring-offset-width)) var(--tw-ring-color)`,
        'box-shadow': `var(--tw-ring-offset-shadow), var(--tw-ring-shadow), var(--tw-shadow)`,
      },
      [[
        `[type='checkbox']:checked`,
        `[type='radio']:checked`,
        `.dark [type='checkbox']:checked`,
        `.dark [type='checkbox']:indeterminate`,
        `.dark [type='radio']:checked`,
      ]]: {
        'border-color': `transparent`,
        'background-color': `currentColor`,
        'background-size': `0.65rem 0.65rem`,
        'background-position': `center`,
        'background-repeat': `no-repeat`,
      },
      [`[type='checkbox']:checked`]: {
        'background-image': `url("${svgToTinyDataUri(
          `<svg aria-hidden="true" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 16 12">
            <path stroke="white" stroke-linecap="round" stroke-linejoin="round" stroke-width="3" d="M1 5.917 5.724 10.5 15 1.5"/>
          </svg>`
        )}")`,
        'background-repeat': `no-repeat`,
        'background-size': `0.65rem 0.65rem`,
        'print-color-adjust': `exact`,
      },
      [`[type='radio']:checked`]: {
        'background-image': `url("${svgToTinyDataUri(
          `<svg viewBox="0 0 16 16" fill="white" xmlns="http://www.w3.org/2000/svg"><circle cx="8" cy="8" r="3"/></svg>`
        )}")`,
        'background-size': `1rem 1rem`,
      },
      [`.dark [type='radio']:checked`]: {
        'background-image': `url("${svgToTinyDataUri(
          `<svg viewBox="0 0 16 16" fill="white" xmlns="http://www.w3.org/2000/svg"><circle cx="8" cy="8" r="3"/></svg>`
        )}")`,
        'background-size': `1rem 1rem`,
      },
      [`[type='checkbox']:indeterminate`]: {
        'background-image': `url("${svgToTinyDataUri(
          `<svg xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 16 16"><path stroke="white" stroke-linecap="round" stroke-linejoin="round" stroke-width="4" d="M2 8h12"/></svg>`
        )}")`,
        'background-color': `currentColor`,
        'border-color': `transparent`,
        'background-position': `center`,
        'background-repeat': `no-repeat`,
        'background-size': `.65rem .65rem`,
        'print-color-adjust': `exact`,
      },
      [[
        `[type='checkbox']:indeterminate:hover`,
        `[type='checkbox']:indeterminate:focus`,
      ]]: {
        'border-color': 'transparent',
        'background-color': 'currentColor',
      },
      [`[type='file']`]: {
        background: 'unset',
        'border-color': 'inherit',
        'border-width': '0',
        'border-radius': '0',
        padding: '0',
        'font-size': 'unset',
        'line-height': 'inherit',
      },
      [`[type='file']:focus`]: {
        outline: `1px auto inherit`,
      },
      [[`input[type=file]::file-selector-button`]]: {
        color: 'white',
        background: theme('colors.gray.800', colors.gray[800]),
        border: 0,
        'font-weight': theme('fontWeight.medium'),
        'font-size': theme('fontSize.sm'),
        cursor: 'pointer',
        'padding-top': spacing[2.5],
        'padding-bottom': spacing[2.5],
        'padding-inline-start': spacing[8],
        'padding-inline-end': spacing[4],
        'margin-inline-start': '-1rem',
        'margin-inline-end': '1rem',
        '&:hover': {
          background: theme('colors.gray.700', colors.gray[700]),
        },
      },
      [[`.dark input[type=file]::file-selector-button`]]: {
        color: 'white',
        background: theme('colors.gray.600', colors.gray[600]),
        '&:hover': {
          background: theme('colors.gray.500', colors.gray[500]),
        },
      },
      [['.tooltip-arrow', '.tooltip-arrow:before']]: {
        position: 'absolute',
        width: '8px',
        height: '8px',
        background: 'inherit',
      },
      ['.tooltip-arrow']: {
        visibility: 'hidden',
      },
      ['.tooltip-arrow:before']: {
        content: '""',
        visibility: 'visible',
        transform: 'rotate(45deg)',
      },
      [`.tooltip[data-popper-placement^='top'] > .tooltip-arrow`]: {
        bottom: '-4px',
      },
      [`.tooltip[data-popper-placement^='bottom'] > .tooltip-arrow`]: {
        top: '-4px',
      },
      [`.tooltip[data-popper-placement^='left'] > .tooltip-arrow`]: {
        right: '-4px',
      },
      [`.tooltip[data-popper-placement^='right'] > .tooltip-arrow`]: {
        left: '-4px',
      },
      ['.tooltip.invisible > .tooltip-arrow:before']: {
        visibility: 'hidden',
      },
      [['[data-popper-arrow]', '[data-popper-arrow]:before']]: {
        position: 'absolute',
        width: '8px',
        height: '8px',
        background: 'inherit',
      },
      ['[data-popper-arrow]']: {
        visibility: 'hidden',
      },
      ['[data-popper-arrow]:before']: {
        content: '""',
        visibility: 'visible',
        transform: 'rotate(45deg)',
      },
      [`[data-popover][role="tooltip"][data-popper-placement^='top'] > [data-popper-arrow]`]:
      {
        bottom: '-5px',
      },
      [`[data-popover][role="tooltip"][data-popper-placement^='bottom'] > [data-popper-arrow]`]:
      {
        top: '-5px',
      },
      [`[data-popover][role="tooltip"][data-popper-placement^='left'] > [data-popper-arrow]`]:
      {
        right: '-5px',
      },
      [`[data-popover][role="tooltip"][data-popper-placement^='right'] > [data-popper-arrow]`]:
      {
        left: '-5px',
      },
      ['[role="tooltip"].invisible > [data-popper-arrow]:before']: {
        visibility: 'hidden',
      },
      '[type=checkbox]': {
        '@apply w-4 h-4 text-blue-600 bg-gray-100 border-gray-300 rounded-sm focus:ring-blue-500 dark:focus:ring-blue-600 dark:ring-offset-gray-800 focus:ring-2 dark:bg-gray-700 dark:border-gray-600': {}
      },
      '[type=radio]': {
        '@apply w-4 h-4 border-gray-300 focus:ring-2 focus:ring-blue-300 dark:focus:ring-blue-600 dark:focus:bg-blue-600 dark:bg-gray-700 dark:border-gray-600': {}
      },
      [['[type=datetime-local]', '[type=month]', '[type=week]', '[type=search]', '[type=date]', '[type=email]', '[type=number]', '[type=password]', '[type=tel]', '[type=text]', '[type=time]', '[type=url]', 'select', 'textarea']]: {
        '@apply bg-gray-50 border border-gray-300 text-gray-900 rounded-md focus:ring-blue-500 focus:border-blue-500 w-full dark:bg-gray-700 dark:border-gray-600 dark:placeholder-gray-400 dark:text-white dark:focus:ring-blue-500 dark:focus:border-blue-500': {}
      },
      'a': {
        '@apply text-blue-600 underline underline-offset-[.2rem]': {}
      },
    });
    addComponents({
      '.action-item-button': {
        '@apply py-2 px-3 text-sm font-medium no-underline text-gray-900 focus:outline-hidden bg-white rounded-md border border-gray-200 hover:bg-gray-100 hover:text-blue-700 focus:z-10 focus:ring-4 focus:ring-gray-200 dark:focus:ring-gray-700 dark:bg-gray-800 dark:text-gray-400 dark:border-gray-600 dark:hover:text-white dark:hover:bg-gray-700': {}
      },
      '.index-data-table-toolbar': {
        '@apply flex flex-col lg:flex-row gap-4 mb-4': {}
      },
      '.scopes': {
        '@apply flex flex-wrap gap-1.5': {}
      },
      '.index-button-group': {
        '@apply inline-flex flex-wrap items-stretch rounded-md': {}
      },
      // Prevent double borders when buttons are next to each other
      '.index-button-group > :where(*:not(:first-child))': {
        '@apply -ms-px my-0': {}
      },
      '.index-button': {
        '@apply inline-flex items-center justify-center px-3 py-2 text-sm font-medium no-underline text-gray-900 bg-white border border-gray-200 hover:bg-gray-100 focus:z-10 focus:ring-2 focus:ring-blue-700 focus:text-blue-700 first:rounded-s-md last:rounded-e-md dark:bg-gray-900 dark:border-gray-700 dark:text-gray-100 dark:hover:text-gray-200 dark:hover:bg-gray-800 dark:focus:ring-blue-500 dark:focus:text-white': {}
      },
      '.index-button-selected': {
        '@apply bg-gray-100 hover:bg-gray-100 dark:bg-gray-800 dark:hover:bg-gray-800': {}
      },
      '.scopes-count': {
        '@apply inline-flex items-center justify-center rounded-full bg-indigo-200/80 text-indigo-800 dark:bg-indigo-800 dark:text-indigo-200 px-1.5 py-1 text-xs font-normal ms-2 leading-none': {}
      },
      '.paginated-collection': {
        '@apply border border-gray-200 dark:border-gray-800 rounded-md shadow-xs overflow-hidden': {}
      },
      '.paginated-collection-contents': {
        '@apply overflow-x-auto': {}
      },
      '.paginated-collection-pagination': {
        '@apply p-2 lg:p-3 flex flex-col-reverse lg:flex-row gap-4 items-center justify-between': {}
      },
      '.paginated-collection-footer': {
        '@apply p-3 flex gap-2 items-center justify-between text-sm border-t border-gray-200 dark:border-gray-800': {}
      },
      '.pagination-per-page': {
        '@apply text-sm py-1 pe-7 w-auto w-min': {}
      },
      '.index-as-table': {
        '@apply relative overflow-x-auto': {}
      },
      '.data-table': {
        '@apply w-full text-sm text-gray-800 dark:text-gray-300': {}
      },
      '.data-table :where(thead > tr > th)': {
        '@apply px-3 py-3.5 whitespace-nowrap font-semibold text-start text-xs uppercase border-b text-gray-700 bg-gray-50 dark:bg-gray-950/50 dark:border-gray-800 dark:text-white': {}
      },
      '.data-table :where(thead > tr > th > a)': {
        '@apply text-inherit no-underline inline-flex items-center gap-2': {}
      },
      '.data-table-sorted-icon': {
        '@apply invisible w-[8px] h-[5px]': {}
      },
      ':where(th[data-sort-direction]) .data-table-sorted-icon': {
        '@apply visible': {}
      },
      ':where(th[data-sort-direction="asc"]) .data-table-sorted-icon': {
        '@apply rotate-180': {}
      },
      '.data-table :where(tbody > tr)': {
        '@apply border-b dark:border-gray-800': {}
      },
      '.data-table :where(td)': {
        '@apply px-3 py-4': {}
      },
      '.data-table-resource-actions': {
        '@apply flex gap-2': {}
      },
      '.filters-form': {
        '@apply text-sm mb-6': {}
      },
      '.filters-form-title': {
        '@apply text-gray-700 dark:text-gray-200 font-bold text-lg mb-4': {}
      },
      '.filters-form :where(.label)': {
        '@apply block mb-1.5 text-sm': {}
      },
      '.filters-form-input-group': {
        '@apply grid grid-cols-2 gap-2': {}
      },
      '.filters-form-field': {
        '@apply mb-4': {}
      },
      '.filters-form-field :where(.choices > label)': {
        '@apply flex gap-2 items-center mb-1': {}
      },
      '.filters-form-buttons': {
        '@apply flex gap-2 items-center': {}
      },
      '.filters-form-submit': {
        '@apply min-w-[6rem] font-bold text-white bg-blue-600 hover:bg-blue-700 focus:ring-4 focus:outline-hidden focus:ring-blue-300 rounded-md px-3 py-2 text-center dark:bg-blue-600 dark:hover:bg-blue-700 dark:focus:ring-blue-800 cursor-pointer': {}
      },
      '.filters-form-clear': {
        '@apply rounded-md px-3 py-2 font-semibold text-gray-700 hover:bg-gray-100 no-underline dark:text-gray-400 dark:hover:bg-inherit dark:hover:text-gray-100 dark:focus:ring-blue-800': {}
      },
      '.active-filters-title': {
        '@apply text-gray-700 dark:text-gray-200 font-bold text-lg mb-4': {}
      },
      '.active-filters-list': {
        '@apply ps-5 list-disc space-y-1 text-gray-700 dark:text-gray-200': {}
      },
      '.batch-actions-dropdown': {
        '@apply relative': {}
      },
      '.batch-actions-dropdown-toggle': {
        '@apply transition-opacity rounded-md inline-flex items-center justify-center gap-2 px-3 py-2 text-sm font-medium text-gray-900 bg-white border border-gray-200 hover:bg-gray-100 hover:text-blue-700 focus:z-10 focus:ring-2 focus:ring-blue-700 focus:text-blue-700 dark:bg-gray-700 dark:border-gray-600 dark:text-white dark:hover:text-white dark:hover:bg-gray-600 dark:focus:ring-blue-500 dark:focus:text-white disabled:text-gray-400 disabled:border-gray-200/70 dark:disabled:bg-gray-900 dark:disabled:text-gray-700 dark:disabled:border-gray-800 disabled:pointer-events-none': {}
      },
      '.batch-actions-dropdown-arrow': {
        '@apply w-2.5 h-2.5': {}
      },
      '.batch-actions-dropdown-menu': {
        '@apply z-10 hidden min-w-[7rem] bg-white rounded-md shadow-lg ring-1 ring-black/5 focus:outline-hidden dark:bg-gray-700 py-1 text-sm text-gray-700 dark:text-gray-200': {}
      },
      '.batch-actions-dropdown-menu :where(li > a)': {
        '@apply block px-2.5 py-2 no-underline text-gray-700 hover:bg-gray-100 hover:text-gray-900 dark:text-white dark:hover:bg-gray-600 dark:hover:text-white': {}
      },
      '.panel': {
        '@apply mb-6 border border-gray-200 rounded-md shadow-xs dark:border-gray-800': {}
      },
      '.panel-title': {
        '@apply font-bold bg-gray-100 dark:bg-gray-900 rounded-t-md p-3': {}
      },
      '.panel-body': {
        '@apply py-5 px-3': {}
      },
      '.attributes-table': {
        '@apply overflow-hidden mb-6 border border-gray-200 rounded-md shadow-xs dark:border-gray-800': {}
      },
      '.attributes-table > :where(table)': {
        '@apply w-full text-sm text-gray-800 dark:text-gray-300': {}
      },
      '.attributes-table :where(tbody > tr)': {
        '@apply border-b dark:border-gray-800': {}
      },
      '.attributes-table :where(tbody > tr > th)': {
        '@apply w-32 sm:w-40 text-start text-xs text-gray-700 uppercase bg-gray-50 dark:bg-gray-800/60 dark:text-gray-300': {}
      },
      '.attributes-table :where(tbody > tr > th, tbody > tr > td)': {
        '@apply p-3': {}
      },
      '.attributes-table-empty-value': {
        '@apply text-gray-400/50 dark:text-gray-700/60 text-xs uppercase font-semibold': {}
      },
      '.status-tag': {
        '@apply bg-gray-200 text-gray-600 dark:bg-gray-700 dark:text-gray-400 inline-flex items-center rounded-full text-sm font-medium px-2.5 py-0.5 whitespace-nowrap': {}
      },
      '.status-tag:where([data-status=yes])': {
        '@apply bg-green-100 text-green-800 dark:bg-green-900 dark:text-green-300': {}
      },
      '.tabs-nav': {
        '@apply flex flex-wrap mb-2 text-sm font-medium text-center border-b border-gray-200 dark:border-gray-700': {}
      },
      '.tabs-nav > :where(a)': {
        '@apply block p-4 border-b-2 border-transparent rounded-t-md hover:text-gray-600 dark:hover:text-gray-300 no-underline': {}
      },
      '.tabs-content': {
        '@apply p-4 mb-6': {}
      },
      // Forms
      '.formtastic': {
        '@apply text-sm': {}
      },
      '.formtastic :where(.fieldset-title, .has-many-fields-title)': {
        '@apply block w-full mb-3 border-b font-bold text-lg': {}
      },
      '.formtastic :where(.label)': {
        '@apply block mb-1.5': {}
      },
      '.formtastic :where(.label abbr)': {
        '@apply ms-1 no-underline': {}
      },
      '.formtastic :where(.input)': {
        '@apply py-3': {}
      },
      '.formtastic :where(.choice)': {
        '@apply mb-1': {}
      },
      '.formtastic :where(.boolean label, .choice label)': {
        '@apply flex gap-2 items-center': {}
      },
      '.formtastic :where(.fragments-group)': {
        '@apply inline-flex flex-wrap gap-1': {}
      },
      '.formtastic :where(.fragment label)': {
        '@apply sr-only': {}
      },
      '.formtastic :where(.inline-hints)': {
        '@apply text-gray-500 mt-2': {}
      },
      '.formtastic :where(.errors)': {
        '@apply p-4 mb-6 rounded-md space-y-2 bg-red-50 text-red-800 dark:bg-red-800 dark:text-red-300': {}
      },
      '.formtastic :where(.errors > li)': {
        '@apply list-disc ms-4': {}
      },
      '.formtastic :where(.inline-errors)': {
        '@apply font-bold mt-2 text-red-600 dark:text-red-300': {}
      },
      '.formtastic :where(.error [type=email], .error [type=number], .error [type=password], .error [type=tel], .error [type=text], .error [type=url], .error textarea)': {
        '@apply border-red-500': {}
      },
      '.formtastic :where(.buttons, .actions)': {
        '@apply mt-3': {}
      },
      '.formtastic :where(.actions > ol)': {
        '@apply flex items-center gap-6': {}
      },
      '.formtastic :where([type=submit], [type=button], button)': {
        '@apply font-bold text-white bg-blue-600 hover:bg-blue-700 focus:ring-4 focus:outline-hidden focus:ring-blue-300 rounded-lg px-4 py-2 text-center dark:bg-blue-600 dark:hover:bg-blue-700 dark:focus:ring-blue-800 cursor-pointer': {}
      },
      '.formtastic :where(.actions .cancel-link)': {
        '@apply font-semibold leading-6 text-gray-900 dark:text-white no-underline': {}
      },
      '.formtastic :where(.has-many-add)': {
        '@apply inline-block py-3': {}
      },
      '.formtastic :where(.has-many-container)': {
        '@apply space-y-8': {}
      },
      '.formtastic :where(.has-many-fields)': {
        '@apply ps-3 border-s-4 border-s-gray-200 dark:border-s-gray-700': {}
      }
    });
  }
)

plugin.js にはtailwindのcssクラスを付与するメソッドがあるのですがそのcssクラスがv3のままなのでエラーが出ていたので修正しました。

次にapp/asesets/stylesheets/ 内に active_admin.css (無ければ作成して下さい。)

app/asesets/stylesheets/active_admin.css

@import "tailwindcss";

@config "tailwind-active_admin.config.jsへのパスを記述";
/* @config "../../../config/tailwind-active_admin.config.js"; */

lib/tasks/active_admin.rake を作成

lib/tasks/active_admin.rake

namespace :active_admin do
  desc "Build Active Admin Tailwind stylesheets"
  task build: :environment do
    command = [
      "tailwindcss",
      "-i", Rails.root.join("app/assets/stylesheets/active_admin.css").to_s,
      "-o", Rails.root.join("app/assets/builds/active_admin.css").to_s
    ]
    system(*command, exception: true)
  end

  desc "Watch Active Admin Tailwind stylesheets"
  task watch: :environment do
    command = [
      "tailwindcss",
      "-w",
      "-i", Rails.root.join("app/assets/stylesheets/active_admin.css").to_s,
      "-o", Rails.root.join("app/assets/builds/active_admin.css").to_s,
      "-m"
    ]

    system(*command, exception: true)
  end
end

Rake::Task["assets:precompile"].enhance([ "active_admin:build" ])

最後にProcfileに追加

Procfile.dev

web: bin/rails server -b 0.0.0.0
css: bin/rails tailwindcss:watch
active_admin: bin/rails active_admin:watch

これで ./bin/dev すればActiveadmin v4が！

gactiveadminの4.0.0.beta15から変更したものをgithubに上げているのでそちらでも
gemfileに
gem "activeadmin", github: "mammosu/activeadmin_v4", branch: "rails8-tailwind4-activeadmin-4"
を追加すれば使用できると思います。