Error while starting AppServer. Required port '4000' already in use

Error while starting AppServer. Required port '4000' already in use

I faced this error while registering database to ArcGIS server, well this can be resolved by opening 

1. Control Panel 
2. System and Security
3. Administrative Tools
4. Services

On Services window find the ArcGIS server service and stop it. 

and after few seconds start it or restart it for automatic restart. Then again try and you'll see that error is removed.

Reason for this.

ArcGIS server uses javaw.exe and it got Hang, This was keeping port 4000 busy independently from ArcGIS server. So it said that Error while starting AppServer. Required port '4000' already in use.