well, it selects the number based on the already used COM's, so if 3 COM's are used, the next one will be COM4.. First it connects to COM3, but after a while to COM4, that means the arduino connection is somehow still present/active in the system, but fails to be recognized, so it assigns the next available port number to it, which is COM4. The strange thing is that COM3 doesn't show up anywhere as connected COM port, it just disappears.