michinyon:
processing.app.SerialException: Serial port 'COM4' already in use. Try quiting any programs that may be using it.
This is the error message I am getting today. according to windows task manager, there is one instance of avrdude.exe running.
That happens if you try to upload while an upload is still in-progress. Wait for an upload to finish or timeout before trying again. You can use Task Manager to kill running instances of avrdude to speed the recovery process.