diff --git a/Formula/m4.rb b/Formula/m4.rb
new file mode 100644
index 0000000..1f7d999
--- /dev/null
+++ b/Formula/m4.rb
@@ -0,0 +1,73 @@
+require (Tap.fetch("automattic", "kandelo-homebrew").path/"Kandelo/formula_support/kandelo_formula_support").to_s
+
+class M4 < Formula
+  include KandeloFormulaSupport
+
+  desc "GNU macro processor for Kandelo"
+  homepage "https://www.gnu.org/software/m4/"
+  url "https://ftpmirror.gnu.org/gnu/m4/m4-1.4.21.tar.xz"
+  mirror "https://ftp.gnu.org/gnu/m4/m4-1.4.21.tar.xz"
+  sha256 "f25c6ab51548a73a75558742fb031e0625d6485fe5f9155949d6486a2408ab66"
+  license "GPL-3.0-or-later"
+
+  depends_on "automattic/kandelo-homebrew/dash"
+
+  skip_clean "bin/m4"
+
+  # Gnulib identifies musl from the GNU host tuple but assumes that every
+  # musl compiler also defines __linux__. Kandelo intentionally does not.
+  patch :DATA
+
+  def install
+    kandelo_require_arch!("wasm32")
+
+    kandelo_wasm_build do
+      # The SDK site owns target facts; this gnulib runtime probe is package-specific.
+      ENV["gl_cv_func_strerror_0_works"] = "yes"
+
+      system kandelo_configure, *kandelo_std_configure_args,
+        "--disable-nls",
+        "--disable-dependency-tracking",
+        "--with-syscmd-shell=dash"
+      system "make", "-j#{ENV.make_jobs}"
+
+      system "make", "install"
+    end
+  end
+
+  test do
+    assert_match(/m4(?:\.wasm)? \(GNU M4\) 1\.4\.21$/,
+      kandelo_run_wasm(bin/"m4", ["--version"]))
+
+    dash = testpath/"dash"
+    dash.binwrite((formula_opt_bin("automattic/kandelo-homebrew/dash")/"dash").binread)
+    dash.chmod 0755
+
+    definitions = testpath/"definitions.m4"
+    definitions.write("define(`VALUE', `42')dnl\n")
+    source = <<~M4
+      include(`definitions.m4')dnl
+      define(`triple', `$1-$1-$1')dnl
+      triple(Kandelo):VALUE
+      esyscmd(`printf child-process')
+      ifelse(sysval, `0', `child-ok', `child-failed')
+    M4
+
+    assert_equal "Kandelo-Kandelo-Kandelo:42\nchild-process\nchild-ok\n",
+      kandelo_run_wasm(
+        bin/"m4", [], env: { "KERNEL_CWD" => testpath, "KERNEL_PATH" => testpath }, stdin: source
+      )
+  end
+end
+
+__END__
+diff --git a/lib/getlocalename_l-unsafe.c b/lib/getlocalename_l-unsafe.c
+index 67479be..b212a7b 100644
+--- a/lib/getlocalename_l-unsafe.c
++++ b/lib/getlocalename_l-unsafe.c
+@@ -37 +37 @@
+-#if (__GLIBC__ >= 2 && !defined __UCLIBC__) || (defined __linux__ && HAVE_LANGINFO_H) || defined __CYGWIN__
++#if (__GLIBC__ >= 2 && !defined __UCLIBC__) || ((defined __linux__ || MUSL_LIBC) && HAVE_LANGINFO_H) || defined __CYGWIN__
+@@ -468 +468 @@ getlocalename_l_unsafe (int category, locale_t locale)
+-#elif defined __linux__ && HAVE_LANGINFO_H && defined NL_LOCALE_NAME
++#elif (defined __linux__ || MUSL_LIBC) && HAVE_LANGINFO_H && defined NL_LOCALE_NAME
diff --git a/README.md b/README.md
index 3c97ac8..b62a581 100644
--- a/README.md
+++ b/README.md
@@ -34,7 +34,8 @@ Current migration controls and pilots include:
 - `pcre2`, the Unicode-capable regex library, POSIX wrapper, and upstream CLI tools;
 - `dash`, the dependency-free POSIX shell with instrumented subprocess support;
 - `make`, GNU dependency-driven build automation using the tap's POSIX shell;
-- `ed`, the conforming line editor and restricted editor required by patch workflows.
+- `ed`, the conforming line editor and restricted editor required by patch workflows;
+- `m4`, the GNU macro processor with process-executing builtins backed by the tap's Dash shell.
 
 The SDK is not yet a Homebrew dependency. Trusted builds supply an
 `HOMEBREW_KANDELO_ROOT` checkout containing the SDK, sysroot, kernel, and Node