Boring a motor is often more detrimental than leaving it the way it is. Without getting too technical, the larger the bore, the smaller the cylinder transfer area. Not a considerable loss in performance, but it is worth noting and definitely not worth bragging about. If anything I would stay away from a motor like this because if you do hurt a piston...chances are you can't bore it to fix it.
A 4mm/7mm/10mm/etc is referring to the overall travel that the piston moves up and down. By increasing the stroke of a motor you can greatly increase the performance of the motor.