The gen 4 have a larger mag release, witch is also able to moved to the left side of the grip for lefties. You also have the dual recoil spring assembly. The "baby" gen 3 glocks have those but all the gen 4 models have them. The gen 4 grip has a more aggressive grip that seems to be well liked.
Also chambered round indicator, different angle on the rear of ejection port, it says "gen4" on slide, magazine is ambidextrous, magazine follower is 6th gen on 4th gen Glock, changeable backstraps on gen4... That's all I can think of.