Bump up device limit to 4

// FREEBIE

src/main/java/org/whispersystems/textsecuregcm/controllers/DeviceController.java

@@ -62,7 +62,7 @@ public class DeviceController {
 
   private final Logger logger = LoggerFactory.getLogger(DeviceController.class);
 
-  private static final int MAX_DEVICES = 3;
+  private static final int MAX_DEVICES = 4;
 
   private final PendingDevicesManager pendingDevices;
   private final AccountsManager       accounts;