Other may scoff at me, but I am a believer in staying off of the first four com ports. Pointing devices (i.e. touchpad, mouse) can still use legacy ports (1,2) and in the old legacy format com ports 3,4 are just extensions of 1,2.
Anyhow, try moving your Mega2560 to Com5 in the Device Manger, Ports, Properties->Port Settings->Advanced...