diff --git a/runtime/lua/vim/filetype.lua b/runtime/lua/vim/filetype.lua
index 6fc984121b..fa7ac53799 100644
--- a/runtime/lua/vim/filetype.lua
+++ b/runtime/lua/vim/filetype.lua
@@ -734,13 +734,10 @@ local extension = {
   luau = 'luau',
   lrc = 'lyrics',
   m = detect.m,
-  at = 'm4',
+  at = 'config',
   mc = detect.mc,
   quake = 'm3quake',
-  m4 = function(path, _bufnr)
-    local pathl = path:lower()
-    return not (pathl:find('html%.m4$') or pathl:find('fvwm2rc')) and 'm4' or nil
-  end,
+  m4 = detect.m4,
   eml = 'mail',
   mk = detect.make,
   mak = detect.make,
@@ -2493,7 +2490,6 @@ local pattern = {
       end
     end,
     ['^hg%-editor%-.*%.txt$'] = 'hgcommit',
-    ['%.html%.m4$'] = 'htmlm4',
     ['^JAM.*%.'] = starsetf('jam'),
     ['^Prl.*%.'] = starsetf('jam'),
     ['^${HOME}/.*/Code/User/.*%.json$'] = 'jsonc',
diff --git a/runtime/lua/vim/filetype/detect.lua b/runtime/lua/vim/filetype/detect.lua
index f20f2a0ecd..a8ef5c0c47 100644
--- a/runtime/lua/vim/filetype/detect.lua
+++ b/runtime/lua/vim/filetype/detect.lua
@@ -1026,6 +1026,42 @@ function M.m(_, bufnr)
   end
 end
 
+--- @type vim.filetype.mapfn
+function M.m4(path, bufnr)
+  local fname = fn.fnamemodify(path, ':t')
+  path = fn.fnamemodify(path, ':p:h')
+
+  -- Case 0: canonical Autoconf file
+  if fname == 'aclocal.m4' then
+    return 'config'
+  end
+
+  -- Case 1: html.m4
+  if fname:find('html%.m4$') then
+    return 'htmlm4'
+  end
+
+  -- Case 2: repo heuristic (nearby configure.ac)
+  if
+    fn.filereadable(path .. '/../configure.ac') ~= 0
+    or fn.filereadable(path .. '/configure.ac') ~= 0
+  then
+    return 'config'
+  end
+
+  -- Case 3: content heuristic (scan first ~200 lines)
+  -- Signals:
+  --   - Autoconf macro prefixes: AC_/AM_/AS_/AU_/AT_
+  for _, line in ipairs(getlines(bufnr, 1, 200)) do
+    if line:find('^%s*A[CMSUT]_') then
+      return 'config'
+    end
+  end
+
+  -- Case 4: default to POSIX M4
+  return 'm4'
+end
+
 --- @param contents string[]
 --- @return string?
 local function m4(contents)
diff --git a/test/old/testdir/test_filetype.vim b/test/old/testdir/test_filetype.vim
index 502f8fc320..248ef9bb4a 100644
--- a/test/old/testdir/test_filetype.vim
+++ b/test/old/testdir/test_filetype.vim
@@ -1,5 +1,10 @@
 " Test :setfiletype
 
+" Make VIMRUNTIME and &rtp absolute.
+" Otherwise, a :lcd inside a test would break the relative ../../runtime path.
+let $VIMRUNTIME = fnamemodify($VIMRUNTIME, ':p')
+let &rtp = join(map(split(&rtp, ','), 'fnamemodify(v:val, ":p")'), ',')
+
 func Test_backup_strip()
   filetype on
   let fname = 'Xdetect.js~~~~~~~~~~~'
@@ -186,7 +191,7 @@ func s:GetFilenameChecks() abort
     \ 'coco': ['file.atg'],
     \ 'conaryrecipe': ['file.recipe'],
     \ 'conf': ['auto.master', 'file.conf', 'texdoc.cnf', '.x11vncrc', '.chktexrc', '.ripgreprc', 'ripgreprc', 'file.ctags'],
-    \ 'config': ['configure.in', 'configure.ac', '/etc/hostname.file', 'any/etc/hostname.file'],
+    \ 'config': ['/etc/hostname.file', 'any/etc/hostname.file', 'configure.in', 'configure.ac', 'alocal.m4', 'file.at'],
     \ 'confini': ['pacman.conf', 'paru.conf', 'mpv.conf', 'any/.aws/config', 'any/.aws/credentials', 'file.nmconnection',
     \             'any/.gnuradio/grc.conf', 'any/gnuradio/config.conf', 'any/gnuradio/conf.d/modtool.conf'],
     \ 'context': ['tex/context/any/file.tex', 'file.mkii', 'file.mkiv', 'file.mkvi', 'file.mkxl', 'file.mklx'],
@@ -463,7 +468,7 @@ func s:GetFilenameChecks() abort
     \            'any/m17n-db/file.ali', 'any/m17n-db/file.cs', 'any/m17n-db/file.dir', 'any/m17n-db/FLT/file.flt', 'any/m17n-db/file.fst', 'any/m17n-db/LANGDATA/file.lnm', 'any/m17n-db/file.mic', 'any/m17n-db/MIM/file.mim', 'any/m17n-db/file.tbl'],
     \ 'm3build': ['m3makefile', 'm3overrides'],
     \ 'm3quake': ['file.quake', 'cm3.cfg'],
-    \ 'm4': ['file.at', '.m4_history'],
+    \ 'm4': ['.m4_history'],
     \ 'mail': ['snd.123', '.letter', '.letter.123', '.followup', '.article', '.article.123', 'pico.123', 'mutt-xx-xxx', 'muttng-xx-xxx', 'ae123.txt', 'file.eml', 'reportbug-file'],
     \ 'mailaliases': ['/etc/mail/aliases', '/etc/aliases', 'any/etc/aliases', 'any/etc/mail/aliases'],
     \ 'mailcap': ['.mailcap', 'mailcap'],
@@ -1146,6 +1151,43 @@ endfunc
 " Keep sorted.
 """""""""""""""""""""""""""""""""""""""""""""""""
 
+" Since dist#ft#FTm4() looks around for configure.ac
+" the test needs to isolate itself in a fresh temporary project tree,
+" so that no configure.ac from another test (or from the repo root)
+" accidentally influences detection.
+func Test_autoconf_file()
+  filetype on
+  " Make a fresh sandbox far away from any configure.ac
+  let save_cwd = getcwd()
+  call mkdir('Xproj_autoconf/a/b', 'p')
+  execute 'lcd Xproj_autoconf/a/b'
+
+  try
+    call writefile(['AC_CHECK_HEADERS([stdio.h])'], 'foo.m4', 'D')
+    split foo.m4
+    call assert_equal('config', &filetype)
+    bwipe!
+
+    call writefile(['AS_IF([true], [:])'], 'bar.m4', 'D')
+    split bar.m4
+    call assert_equal('config', &filetype)
+    bwipe!
+
+    call writefile(['AC_INIT([foo],[1.0])'], 'configure.ac')
+    call mkdir('m4', 'p')
+    call writefile([], 'm4/empty.m4', 'D')
+    split m4/empty.m4
+    call assert_equal('config', &filetype)
+    bwipe!
+  finally
+    call delete('m4', 'rf')
+    call delete('configure.ac')
+    execute 'lcd' fnameescape(save_cwd)
+    call delete('Xproj_autoconf', 'rf')
+    filetype off
+  endtry
+endfunc
+
 func Test_bas_file()
   filetype on
 
@@ -1891,6 +1933,30 @@ func Test_m_file()
   filetype off
 endfunc
 
+" Since dist#ft#FTm4() looks around for configure.ac
+" the test needs to isolate itself in a fresh temporary project tree,
+" so that no configure.ac from another test (or from the repo root)
+" accidentally influences detection.
+func Test_m4_file()
+  filetype on
+  let save_cwd = getcwd()
+
+  " Make a fresh sandbox far away from any configure.ac
+  call mkdir('Xsandbox/level1/level2', 'p')
+  execute 'lcd Xsandbox/level1/level2'
+
+  try
+    call writefile(["define(`FOO', `bar')", "FOO"], 'plain.m4', 'D')
+    split plain.m4
+    call assert_equal('m4', &filetype)
+    bwipe!
+  finally
+    execute 'lcd' fnameescape(save_cwd)
+    call delete('Xsandbox', 'rf')
+    filetype off
+  endtry
+endfunc
+
 func Test_mod_file()
   filetype on