diff --git a/src/EleventyExtensionMap.js b/src/EleventyExtensionMap.js
index 051dc7547..62d140d90 100644
--- a/src/EleventyExtensionMap.js
+++ b/src/EleventyExtensionMap.js
@@ -76,6 +76,8 @@ class EleventyExtensionMap {
 	// on paths found from the file system glob search.
 	// TODO: Method name might just need to be renamed to something more accurate.
 	isFullTemplateFilePath(path) {
+		path = (path || "").toLowerCase();
+
 		for (let extension of this.validTemplateLanguageKeys) {
 			if (path.endsWith(`.${extension}`)) {
 				return true;
@@ -97,6 +99,8 @@ class EleventyExtensionMap {
 	}
 
 	getValidExtensionsForPath(path) {
+		path = (path || "").toLowerCase();
+
 		let extensions = new Set();
 		for (let extension in this.extensionToKeyMap) {
 			if (path.endsWith(`.${extension}`)) {
@@ -242,8 +246,10 @@ class EleventyExtensionMap {
 	}
 
 	removeTemplateExtension(path) {
+		let normalizedPath = (path || "").toLowerCase();
+
 		for (let extension in this.extensionToKeyMap) {
-			if (path === extension || path.endsWith("." + extension)) {
+			if (normalizedPath === extension || normalizedPath.endsWith("." + extension)) {
 				return path.slice(
 					0,
 					path.length - 1 - extension.length < 0 ? 0 : path.length - 1 - extension.length,
diff --git a/src/UserConfig.js b/src/UserConfig.js
index 4c6d58693..c6e674820 100644
--- a/src/UserConfig.js
+++ b/src/UserConfig.js
@@ -928,6 +928,8 @@ class UserConfig {
 		}
 
 		for (let extension of extensions) {
+			extension = extension.trim().toLowerCase();
+
 			if (this.extensionConflictMap[extension]) {
 				throw new Error(
 					`An attempt was made to override the "${extension}" template syntax twice (via the \`addExtension\` configuration API). A maximum of one override is currently supported.`,
@@ -944,6 +946,8 @@ class UserConfig {
 				},
 				options,
 			);
+			extensionOptions.key = extensionOptions.key.toLowerCase();
+			extensionOptions.extension = extensionOptions.extension.toLowerCase();
 
 			if (extensionOptions.key !== extensionOptions.extension) {
 				extensionOptions.aliasKey = extensionOptions.extension;
diff --git a/test/EleventyExtensionMapTest.js b/test/EleventyExtensionMapTest.js
index 03f2b1ca1..b75698106 100644
--- a/test/EleventyExtensionMapTest.js
+++ b/test/EleventyExtensionMapTest.js
@@ -38,6 +38,38 @@ test("Keys are mapped to lower case", async (t) => {
   t.deepEqual(map.getGlobs("."), ["./**/*.{liquid,pug,njk}"]);
 });
 
+test("Custom addExtension keys are mapped to lower case", async (t) => {
+  let cfg = new TemplateConfig();
+  cfg.userConfig.addExtension("vZome", {
+    compile: function(content) {
+      return () => content;
+    }
+  });
+
+  let map = await getExtensionMap(["vZome"], cfg);
+
+  t.deepEqual(map.getValidGlobs("."), ["./**/*.vzome"]);
+  t.true(map.hasEngine("component.vZome"));
+  t.is(map.getKey("component.vZome"), "vzome");
+  t.true(map.isFullTemplateFilePath("component.vZome"));
+  t.deepEqual(map.getValidExtensionsForPath("component.vZome"), ["vzome"]);
+  t.is(map.removeTemplateExtension("component.vZome"), "component");
+});
+
+test("Custom addExtension keys detect mixed-case duplicates", (t) => {
+  let cfg = new TemplateConfig();
+  cfg.userConfig.addExtension("vZome", {});
+
+  let e = t.throws(() => {
+    cfg.userConfig.addExtension("VZOME", {});
+  });
+
+  t.is(
+    e.message,
+    'An attempt was made to override the "vzome" template syntax twice (via the `addExtension` configuration API). A maximum of one override is currently supported.',
+  );
+});
+
 test("Pruned globs", async (t) => {
   let map = await getExtensionMap(["liquid", "njk", "png"]);
   t.deepEqual(map.getPassthroughCopyGlobs("."), ["./**/*.png"]);
diff --git a/test/EleventyTest.js b/test/EleventyTest.js
index ecae5c44f..671dd85d9 100644
--- a/test/EleventyTest.js
+++ b/test/EleventyTest.js
@@ -1622,6 +1622,26 @@ test("permalink on custom template lang, issue #3619", async (t) => {
 }`);
 });
 
+test("Mixed-case custom template extensions are discovered, issue #4224", async (t) => {
+  let elev = new Eleventy("./test/stubs-4224/", undefined, {
+    config: function(eleventyConfig) {
+      eleventyConfig.addTemplateFormats("vZome");
+      eleventyConfig.addExtension("vZome", {
+        compile: function(str) {
+          return function() {
+            return `Rendered ${str.trim()}`;
+          };
+        },
+      });
+    },
+  });
+  elev.disableLogger();
+
+  let results = await elev.toJSON();
+  t.is(results.length, 1);
+  t.is(results[0].content, "Rendered mixed-case custom extension");
+});
+
 test("Template data throws error when tags is not an Array or String #1791", async (t) => {
   let elev = new Eleventy("./test/noop/", "./test/noop/_site", {
     config: function (eleventyConfig) {
diff --git a/test/stubs-4224/index.vZome b/test/stubs-4224/index.vZome
new file mode 100644
index 000000000..fef6ce908
--- /dev/null
+++ b/test/stubs-4224/index.vZome
@@ -0,0 +1 @@
+mixed-case custom extension