diff --git a/src/cursors.ts b/src/cursors.ts
index 9f6ee81..724f74a 100644
--- a/src/cursors.ts
+++ b/src/cursors.ts
@@ -4,7 +4,7 @@
 import { PCM_AUDIO_CODECS } from './codec';
 import { InputAudioTrack, InputTrack, InputVideoTrack } from './input-track';
 import { AudioDecoderWrapper, DecoderWrapper, PacketRetrievalOptions, PcmAudioDecoderWrapper, validatePacketRetrievalOptions, validateTimestamp, VideoDecoderWrapper } from './media-sink';
-import { assert, AsyncMutex3, AsyncMutexLock, CallSerializer2, defer, insertSorted, isFirefox, last, MaybePromise, polyfillSymbolDispose, promiseWithResolvers, ResultValue, Rotation, Yo } from './misc';
+import { assert, AsyncMutex4, AsyncMutexLock, CallSerializer2, defer, insertSorted, isFirefox, last, MaybePromise, polyfillSymbolDispose, promiseWithResolvers, ResultValue, Rotation, Yo } from './misc';
 import { EncodedPacket } from './packet';
 import { AudioSample, clampCropRectangle, CropRectangle, validateCropRectangle, VideoSample } from './sample';
 
@@ -377,9 +377,9 @@ export abstract class SampleCursor<
 	decodedTimestamps: number[] = [];
 	maxDecodedSequenceNumber = -1;
 	pumpTarget: EncodedPacket | null = null;
-	pumpMutex = new AsyncMutex3();
+	pumpMutex = new AsyncMutex4();
 	_closed = false;
-	otherMutex = new AsyncMutex3(); // TODO: THIS IS STILL A BUGGED MUTEX! ASYNC MUTEX 4
+	otherMutex = new AsyncMutex4();
 
 	error: unknown = null;
 	errorSet = false;
@@ -419,9 +419,8 @@ export abstract class SampleCursor<
 				return this.onDecoderError(new Error('Fake decoder error!'));
 			}
 
-			const mutexPromise = this.otherMutex.request();
-			if (mutexPromise) await mutexPromise;
-			using _ = this.otherMutex.lock();
+			using lock = this.otherMutex.lock();
+			if (lock.pending) await lock.ready;
 
 			while (this.decodedTimestamps.length > 0 && this.decodedTimestamps[0]! <= sample.timestamp) {
 				this.decodedTimestamps.shift();
@@ -474,9 +473,8 @@ export abstract class SampleCursor<
 	}
 
 	async onDecoderError(error: unknown) {
-		const mutexPromise = this.otherMutex.request();
-		if (mutexPromise) await mutexPromise;
-		using _ = this.otherMutex.lock();
+		using lock = this.otherMutex.lock();
+		if (lock.pending) await lock.ready;
 
 		await this.closeWithError(error);
 	}
@@ -519,9 +517,8 @@ export abstract class SampleCursor<
 		this.predictedRequests++;
 
 		if (!lock) {
-			const mutexPromise = this.pumpMutex.request();
-			if (mutexPromise) await mutexPromise;
 			lock = this.pumpMutex.lock();
+			if (lock.pending) await lock.ready;
 		}
 
 		this._ensureNotClosed();
@@ -698,9 +695,8 @@ export abstract class SampleCursor<
 	}
 
 	async _nextInternal(res: ResultValue<TransformedSample | null>): Promise<Yo> {
-		const mutexPromise = this.pumpMutex.request();
-		if (mutexPromise) await mutexPromise;
 		using lock = this.pumpMutex.lock();
+		if (lock.pending) await lock.ready;
 
 		this._ensureNotClosed();
 
@@ -850,9 +846,8 @@ export abstract class SampleCursor<
 			this.predictedRequests++;
 			this.packetReader.track.input._openSampleCursors.delete(this);
 
-			const mutexPromise = this.pumpMutex.request();
-			if (mutexPromise) await mutexPromise;
-			using _ = this.pumpMutex.lock();
+			using lock = this.pumpMutex.lock();
+			if (lock.pending) await lock.ready;
 
 			this._closed = true;
 
diff --git a/src/misc.ts b/src/misc.ts
index d577e13..22aba8a 100644
--- a/src/misc.ts
+++ b/src/misc.ts
@@ -896,36 +896,39 @@ export class AsyncMutex2 {
 	}
 }
 
-export type AsyncMutexLock = {
-	release: () => void;
-	[Symbol.dispose]: () => void;
-};
+export interface AsyncMutexLock extends Disposable {
+	readonly pending: boolean;
+	readonly ready: Promise<void> | null;
+	release(): void;
+}
 
-export class AsyncMutex3 {
-	locked = false;
-	resolverQueue: (() => void)[] = [];
+export class AsyncMutex4 {
+	private locked = false;
+	private resolverQueue: (() => void)[] = [];
 
-	lock(): AsyncMutexLock {
-		if (this.locked) {
-			throw new Error('Mutex already locked.');
+	lock() {
+		if (!this.locked) {
+			// Fast path
+			this.locked = true;
+			return this.createLock(false, null);
 		}
 
-		this.locked = true;
+		const { promise, resolve } = promiseWithResolvers();
+		this.resolverQueue.push(resolve);
+
+		return this.createLock(true, promise);
+	}
+
+	private createLock(pending: boolean, ready: Promise<void> | null): AsyncMutexLock {
 		let released = false;
 
 		return {
+			pending,
+			ready,
 			release: () => {
-				if (released) {
-					return;
-				}
+				if (released) return;
 				released = true;
-
-				this.locked = false;
-
-				if (this.resolverQueue.length > 0) {
-					const resolve = this.resolverQueue.shift()!;
-					resolve();
-				}
+				this.dispatch();
 			},
 			[Symbol.dispose]() {
 				this.release();
@@ -933,15 +936,13 @@ export class AsyncMutex3 {
 		};
 	}
 
-	request() {
-		if (!this.locked) {
-			return null;
+	private dispatch() {
+		if (this.resolverQueue.length > 0) {
+			const resolve = this.resolverQueue.shift()!;
+			resolve();
+		} else {
+			this.locked = false;
 		}
-
-		const { promise, resolve } = promiseWithResolvers();
-		this.resolverQueue.push(resolve);
-
-		return promise;
 	}
 }