diff --git a/pkgs/development/compilers/emscripten/default.nix b/pkgs/development/compilers/emscripten/default.nix
index 54386a3ca24e..01d038a687d7 100644
--- a/pkgs/development/compilers/emscripten/default.nix
+++ b/pkgs/development/compilers/emscripten/default.nix
@@ -17,7 +17,7 @@
 
 stdenv.mkDerivation rec {
   pname = "emscripten";
-  version = "3.1.73";
+  version = "4.0.8";
 
   llvmEnv = symlinkJoin {
     name = "emscripten-llvm-${version}";
@@ -33,7 +33,7 @@ stdenv.mkDerivation rec {
     name = "emscripten-node-modules-${version}";
     inherit pname version src;
 
-    npmDepsHash = "sha256-bqxUlxpIH1IAx9RbnaMq4dZW8fy+M/Q02Q7VrW/AKNQ=";
+    npmDepsHash = "sha256-fGlBtXsYOQ5V4/PRPPIpL3nxb+hUAuj9q7Jw0kL7ph0=";
 
     dontBuild = true;
 
@@ -46,7 +46,7 @@ stdenv.mkDerivation rec {
   src = fetchFromGitHub {
     owner = "emscripten-core";
     repo = "emscripten";
-    hash = "sha256-QlC2k2rhF3/Pz+knnrlBDV8AfHHBSlGr7b9Ae6TNsxY=";
+    hash = "sha256-xiqi3SMmlfV7NaA61QZAW7BFHu9xOVN9QMWwwDInBeE=";
     rev = version;
   };
 
@@ -67,8 +67,8 @@ stdenv.mkDerivation rec {
 
     patchShebangs .
 
-    # emscripten 3.1.67 requires LLVM tip-of-tree instead of LLVM 18
-    sed -i -e "s/EXPECTED_LLVM_VERSION = 20/EXPECTED_LLVM_VERSION = 19/g" tools/shared.py
+    # emscripten 4 requires LLVM tip-of-tree instead of LLVM 20
+    sed -i -e "s/EXPECTED_LLVM_VERSION = 21/EXPECTED_LLVM_VERSION = 20.1/g" tools/shared.py
 
     # fixes cmake support
     sed -i -e "s/print \('emcc (Emscript.*\)/sys.stderr.write(\1); sys.stderr.flush()/g" emcc.py
diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index bd8561acc5ee..8366b87702ad 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -2913,7 +2913,7 @@ with pkgs;
   buildEmscriptenPackage = callPackage ../development/em-modules/generic { };
 
   emscripten = callPackage ../development/compilers/emscripten {
-    llvmPackages = llvmPackages_19;
+    llvmPackages = llvmPackages_20;
   };
 
   emscriptenPackages = recurseIntoAttrs (callPackage ./emscripten-packages.nix { });