diff --git a/packages/@webex/internal-plugin-mercury/src/mercury.js b/packages/@webex/internal-plugin-mercury/src/mercury.js
index 129f856dddc..3ecfb58166a 100644
--- a/packages/@webex/internal-plugin-mercury/src/mercury.js
+++ b/packages/@webex/internal-plugin-mercury/src/mercury.js
@@ -21,6 +21,7 @@ import {
 } from './errors';
 
 const normalReconnectReasons = ['idle', 'done (forced)', 'pong not received', 'pong mismatch'];
+const MERCURY_CLOSE_4000_METRIC = 'JS_SDK_MERCURY_CLOSE_4000';
 
 const Mercury = WebexPlugin.extend({
   namespace: 'Mercury',
@@ -882,6 +883,29 @@ const Mercury = WebexPlugin.extend({
     return handlers;
   },
 
+  _submitMercuryClose4000Metric(sessionId, event, options) {
+    const {action, isActiveSocket, messageType} = options;
+
+    try {
+      this.webex.internal.metrics.submitClientMetrics(MERCURY_CLOSE_4000_METRIC, {
+        fields: {
+          action,
+          close_code: event.code,
+          close_reason: event.reason || '',
+          is_active_socket: isActiveSocket,
+          message_type: messageType,
+          session_id: sessionId,
+        },
+        tags: {
+          action,
+          message_type: messageType,
+        },
+      });
+    } catch (error) {
+      this.logger.warn(`${this.namespace}: failed to submit Mercury 4000 close metric`, error);
+    }
+  },
+
   _onclose(sessionId, event, sourceSocket) {
     // I don't see any way to avoid the complexity or statement count in here.
     /* eslint complexity: [0] */
@@ -938,9 +962,28 @@ const Mercury = WebexPlugin.extend({
           break;
         case 4000:
           // metric: disconnect
-          this.logger.info(`${this.namespace}: socket ${sessionId} replaced; will not reconnect`);
-          if (isActiveSocket) this._emit(sessionId, 'offline.replaced', event);
-          // If not active, nothing to do
+          if (reason === 'replaced') {
+            this._submitMercuryClose4000Metric(sessionId, event, {
+              action: 'no_action',
+              isActiveSocket,
+              messageType: 'replaced',
+            });
+            this.logger.info(`${this.namespace}: socket ${sessionId} replaced; will not reconnect`);
+            if (isActiveSocket) this._emit(sessionId, 'offline.replaced', event);
+          } else {
+            this._submitMercuryClose4000Metric(sessionId, event, {
+              action: isActiveSocket ? 'reconnect' : 'ignore_non_active',
+              isActiveSocket,
+              messageType: 'other',
+            });
+            this.logger.info(
+              `${this.namespace}: socket ${sessionId} disconnected with 4000: ${event.reason}; reconnecting`
+            );
+            if (isActiveSocket) {
+              this._emit(sessionId, 'offline.transient', event);
+              this._reconnect(socketUrl, sessionId);
+            }
+          }
           break;
         case 4001:
           // replaced during shutdown
diff --git a/packages/@webex/internal-plugin-mercury/test/unit/spec/mercury-events.js b/packages/@webex/internal-plugin-mercury/test/unit/spec/mercury-events.js
index 42c462a3e77..512e15ceeb3 100644
--- a/packages/@webex/internal-plugin-mercury/test/unit/spec/mercury-events.js
+++ b/packages/@webex/internal-plugin-mercury/test/unit/spec/mercury-events.js
@@ -267,8 +267,14 @@ describe('plugin-mercury', () => {
           },
           {
             code: 4000,
+            reason: 'Replaced',
             action: 'replace',
           },
+          {
+            code: 4000,
+            reason: 'Unexpected close',
+            action: 'reconnect',
+          },
           {
             action: 'close',
           },
diff --git a/packages/@webex/internal-plugin-mercury/test/unit/spec/mercury.js b/packages/@webex/internal-plugin-mercury/test/unit/spec/mercury.js
index e3562a3177f..8aa660d3486 100644
--- a/packages/@webex/internal-plugin-mercury/test/unit/spec/mercury.js
+++ b/packages/@webex/internal-plugin-mercury/test/unit/spec/mercury.js
@@ -1334,6 +1334,129 @@ describe('plugin-mercury', () => {
         });
       });
 
+      describe('#_onclose() with code 4000', () => {
+        let mockSocket, anotherSocket;
+
+        beforeEach(() => {
+          mockSocket = {
+            url: 'ws://active-socket.com',
+            removeAllListeners: sinon.stub(),
+          };
+          anotherSocket = {
+            url: 'ws://old-socket.com',
+            removeAllListeners: sinon.stub(),
+          };
+          mercury.socket = mockSocket;
+          mercury.sockets.set(mercury.defaultSessionId, mockSocket);
+          mercury.sessionWebSocketUrls.set(mercury.defaultSessionId, mockSocket.url);
+          mercury.connected = true;
+          sinon.stub(mercury, '_emit');
+          sinon.stub(mercury, '_reconnect');
+          sinon.stub(mercury, 'unset');
+        });
+
+        afterEach(() => {
+          mercury._emit.restore();
+          mercury._reconnect.restore();
+          mercury.unset.restore();
+        });
+
+        it('should not reconnect when active socket is replaced', () => {
+          const closeEvent = {
+            code: 4000,
+            reason: 'Replaced',
+          };
+
+          mercury._onclose(mercury.defaultSessionId, closeEvent, mockSocket);
+
+          assert.calledOnceWithExactly(
+            webex.internal.metrics.submitClientMetrics,
+            'JS_SDK_MERCURY_CLOSE_4000',
+            {
+              fields: {
+                action: 'no_action',
+                close_code: 4000,
+                close_reason: 'Replaced',
+                is_active_socket: true,
+                message_type: 'replaced',
+                session_id: mercury.defaultSessionId,
+              },
+              tags: {
+                action: 'no_action',
+                message_type: 'replaced',
+              },
+            }
+          );
+          assert.calledWith(mercury._emit, mercury.defaultSessionId, 'offline.replaced', closeEvent);
+          assert.notCalled(mercury._reconnect);
+        });
+
+        it('should reconnect active socket when 4000 has an unknown reason', () => {
+          const closeEvent = {
+            code: 4000,
+            reason: 'Unexpected close',
+          };
+
+          mercury._onclose(mercury.defaultSessionId, closeEvent, mockSocket);
+
+          assert.calledOnceWithExactly(
+            webex.internal.metrics.submitClientMetrics,
+            'JS_SDK_MERCURY_CLOSE_4000',
+            {
+              fields: {
+                action: 'reconnect',
+                close_code: 4000,
+                close_reason: 'Unexpected close',
+                is_active_socket: true,
+                message_type: 'other',
+                session_id: mercury.defaultSessionId,
+              },
+              tags: {
+                action: 'reconnect',
+                message_type: 'other',
+              },
+            }
+          );
+          assert.calledWith(mercury._emit, mercury.defaultSessionId, 'offline.transient', closeEvent);
+          assert.calledWith(
+            mercury._reconnect,
+            mockSocket.url,
+            mercury.defaultSessionId
+          );
+        });
+
+        it('should not reconnect a non-active socket with an unknown 4000 reason', () => {
+          const closeEvent = {
+            code: 4000,
+            reason: 'Unexpected close',
+          };
+
+          mercury._onclose(mercury.defaultSessionId, closeEvent, anotherSocket);
+
+          assert.calledOnceWithExactly(
+            webex.internal.metrics.submitClientMetrics,
+            'JS_SDK_MERCURY_CLOSE_4000',
+            {
+              fields: {
+                action: 'ignore_non_active',
+                close_code: 4000,
+                close_reason: 'Unexpected close',
+                is_active_socket: false,
+                message_type: 'other',
+                session_id: mercury.defaultSessionId,
+              },
+              tags: {
+                action: 'ignore_non_active',
+                message_type: 'other',
+              },
+            }
+          );
+          assert.notCalled(mercury._emit);
+          assert.notCalled(mercury._reconnect);
+          assert.isTrue(mercury.connected);
+        });
+      });
+
       describe('#_onclose() with code 4001 (shutdown replacement)', () => {
         let mockSocket, anotherSocket;