When I connect my Arduino to my laptop and want to get serial data on my system, I get an exception about port access being denied. I am sure about the port number (COM4) and opening and closing ports. sometimes I reconnect the device it works.

>Solution :

Try these out:

• if your Serial monitor on Arduino is not opened, be sure to close it.

• check the port in system management of your system to be sure about
  the port number again.

• be sure about closing your application each time you want to test,
  because running an application will reserve the port and will not let
  other apps use it.