Communicate with Arduino through the COM4 port

Question

I created a code in c# to communicate with the Arduino uno. He has to pass me what the tag passed by the rfid reader and show in a textbox the value.

I added the serialport1.

Code:

        serialPort1.Open();
        string entrada = serialPort1.ReadLine();
        serialPort1.Close();
        txtIDTag.Text = entrada;

Error when executing:

An unhandled Exception of type 'System.Unauthorizedaccessexception' occurred in System.dll

Additional information: The 'COM4' port does not exist.