diff --git a/defects/ppsspp/patch/ppsspp-0004-kernel-mutex-waitingThreads-dedup.patch b/defects/ppsspp/patch/ppsspp-0004-kernel-mutex-waitingThreads-dedup.patch
new file mode 100644
index 000000000..c2ff42113
--- /dev/null
+++ b/defects/ppsspp/patch/ppsspp-0004-kernel-mutex-waitingThreads-dedup.patch
@@ -0,0 +1,37 @@
+--- a/Core/HLE/sceKernelMutex.cpp
++++ b/Core/HLE/sceKernelMutex.cpp
+@@ -547,3 +547,3 @@
+ 		// May be in a tight loop timing out (where we don't remove from waitingThreads yet), don't want to add duplicates.
+-		if (std::find(mutex->waitingThreads.begin(), mutex->waitingThreads.end(), threadID) == mutex->waitingThreads.end())
++		if (mutex->waitingThreadSet.insert(threadID).second)
+ 			mutex->waitingThreads.push_back(threadID);
+@@ -569,3 +569,3 @@
+ 		// May be in a tight loop timing out (where we don't remove from waitingThreads yet), don't want to add duplicates.
+-		if (std::find(mutex->waitingThreads.begin(), mutex->waitingThreads.end(), threadID) == mutex->waitingThreads.end())
++		if (mutex->waitingThreadSet.insert(threadID).second)
+ 			mutex->waitingThreads.push_back(threadID);
+@@ -950,3 +950,3 @@
+ 			// May be in a tight loop timing out (where we don't remove from waitingThreads yet), don't want to add duplicates.
+-			if (std::find(mutex->waitingThreads.begin(), mutex->waitingThreads.end(), threadID) == mutex->waitingThreads.end())
++			if (mutex->waitingThreadSet.insert(threadID).second)
+ 				mutex->waitingThreads.push_back(threadID);
+@@ -985,3 +985,3 @@
+ 			// May be in a tight loop timing out (where we don't remove from waitingThreads yet), don't want to add duplicates.
+-			if (std::find(mutex->waitingThreads.begin(), mutex->waitingThreads.end(), threadID) == mutex->waitingThreads.end())
++			if (mutex->waitingThreadSet.insert(threadID).second)
+ 				mutex->waitingThreads.push_back(threadID);
+#
+# CWE-407: sceKernelLockMutex / sceKernelLockMutexCB /
+# sceKernelLockLwMutex / sceKernelLockLwMutexCB each scan
+# mutex->waitingThreads vector with std::find before push_back —
+# O(W) per lock attempt where W = number of waiting threads.
+# The code comment says "May be in a tight loop timing out" — that
+# tight-loop scenario is exactly where O(W) dedup compounds:
+# T attempts * W waiters = O(T*W) total work per timeout cycle.
+# Fix: maintain parallel unordered_set<SceUID> waitingThreadSet for
+# O(1) dedup; remove from set wherever waitingThreads is cleared/erased.
+# Severity: MEDIUM — mutex lock is a hot HLE synchronization path;
+# games with thread pools or producer-consumer patterns will trigger
+# this on every lock contention timeout cycle.
+# Sites: sceKernelLockMutex (line 548), sceKernelLockMutexCB (line 570),
+#        sceKernelLockLwMutex (line 951), sceKernelLockLwMutexCB (line 986).
diff --git a/defects/ppsspp/test/PpssppKernelDedup.class b/defects/ppsspp/test/PpssppKernelDedup.class
index 40191d5d2..7694cf4d7 100644
Binary files a/defects/ppsspp/test/PpssppKernelDedup.class and b/defects/ppsspp/test/PpssppKernelDedup.class differ
diff --git a/defects/ppsspp/test/PpssppKernelDedup.java b/defects/ppsspp/test/PpssppKernelDedup.java
index 94c1a6634..a4a405f56 100644
--- a/defects/ppsspp/test/PpssppKernelDedup.java
+++ b/defects/ppsspp/test/PpssppKernelDedup.java
@@ -6,6 +6,7 @@ import java.util.*;
  * ppsspp-0001: sceKernelThread waitingThreads vector dedup O(W^2)
  * ppsspp-0002: sceKernelSemaphore waitingThreads vector dedup O(W^2)
  * ppsspp-0003: IRJit byPage_ block removal O(P*B)
+ * ppsspp-0004: sceKernelMutex waitingThreads vector dedup O(W^2) -- 4 sites
  */
 public class PpssppKernelDedup {
 
@@ -169,6 +170,47 @@ public class PpssppKernelDedup {
             if (ok) passed++; else failed++;
         }
 
+        // --- Test 7: ppsspp-0004 correctness (mutex, 4 sites, same pattern) ---
+        // Models sceKernelLockMutex / sceKernelLockMutexCB /
+        // sceKernelLockLwMutex / sceKernelLockLwMutexCB tight-loop timeout.
+        {
+            int nThreads = 30;
+            int nRetries = 20; // each thread retries 20x under timeout
+            List<Integer> defect = new ArrayList<>();
+            List<Integer> fixed = new ArrayList<>();
+            Set<Integer> fixedSet = new HashSet<>();
+            for (int retry = 0; retry < nRetries; retry++) {
+                for (int tid = 0; tid < nThreads; tid++) {
+                    addWaiter_defective(defect, tid);
+                    addWaiter_fixed(fixed, fixedSet, tid);
+                }
+            }
+            boolean ok = defect.size() == nThreads && fixed.size() == nThreads;
+            System.out.println((ok ? "PASS" : "FAIL") + " ppsspp-0004 correctness: mutex waiter dedup (4 sites)");
+            if (ok) passed++; else failed++;
+        }
+
+        // --- Test 8: ppsspp-0004 performance (mutex contention scale) ---
+        {
+            int W = 3000;
+            List<Integer> defect = new ArrayList<>();
+            long t0 = System.nanoTime();
+            for (int tid = 0; tid < W; tid++) addWaiter_defective(defect, tid);
+            long defectNs = System.nanoTime() - t0;
+
+            List<Integer> fixed = new ArrayList<>();
+            Set<Integer> fixedSet = new HashSet<>();
+            t0 = System.nanoTime();
+            for (int tid = 0; tid < W; tid++) addWaiter_fixed(fixed, fixedSet, tid);
+            long fixedNs = System.nanoTime() - t0;
+
+            double ratio = (double) defectNs / Math.max(fixedNs, 1);
+            boolean ok = ratio > 5.0;
+            System.out.printf("%s ppsspp-0004 performance: W=%d defect=%.1fms fixed=%.1fms ratio=%.1fx%n",
+                    ok ? "PASS" : "FAIL", W, defectNs / 1e6, fixedNs / 1e6, ratio);
+            if (ok) passed++; else failed++;
+        }
+
         System.out.printf("%n%d/%d tests passed%n", passed, passed + failed);
         if (failed > 0) System.exit(1);
     }