diff --git a/backend/src/langs.ts b/backend/src/langs.ts
index 814ca72..29bb602 100644
--- a/backend/src/langs.ts
+++ b/backend/src/langs.ts
@@ -2071,6 +2071,17 @@ KTHXBYE
 `,
     skip: ["lsp"],
   },
+  m4: {
+    name: "m4",
+    repl: "echo 'Reading from stdin...' >&2; m4",
+    input: `eval(123 * 234)`,
+    main: "main.m4",
+    run: "echo 'Reading from stdin...' >&2; m4 main.m4 -",
+    scope: {
+      code: "define(`x', eval(123 * 234))",
+    },
+    template: "errprint(`Hello, world!')\n",
+  },
   make: {
     aliases: [
       "gmake",
diff --git a/scripts/docker-install-phase3c.bash b/scripts/docker-install-phase3c.bash
index b02aa5e..5b7201a 100755
--- a/scripts/docker-install-phase3c.bash
+++ b/scripts/docker-install-phase3c.bash
@@ -6,6 +6,9 @@ set -x
 
 packages="
 
+# m4
+m4
+
 # MariaDB
 libtinfo5