AP_SerialManager: changed default for SERIAL4_PROTOCOL to 5

this makes it a documented protocol number. No functionality change

libraries/AP_SerialManager/AP_SerialManager.cpp

@@ -90,7 +90,7 @@ const AP_Param::GroupInfo AP_SerialManager::var_info[] = {
     // @Description: Control what protocol Serial4 port should be used for. Note that the Frsky options require external converter hardware. See the wiki for details.
     // @Values: -1:None, 1:GCS Mavlink, 3:Frsky D-PORT, 4:Frsky S-PORT, 5:GPS, 7:Alexmos Gimbal Serial, 8:SToRM32 Gimbal Serial, 9:Lidar
     // @User: Standard
-    AP_GROUPINFO("4_PROTOCOL",  7, AP_SerialManager, state[4].protocol, SerialProtocol_GPS2),
+    AP_GROUPINFO("4_PROTOCOL",  7, AP_SerialManager, state[4].protocol, SerialProtocol_GPS),
 
     // @Param: 4_BAUD
     // @DisplayName: Serial 4 Baud Rate