Glock22

Gen4 has interchangeable back straps and gen3 does not. Gen4 also has a tighter trigger pull and i think the feel of the frame is also different. Other than that its the same.

Has a dual recoil spring assembly. And has a bigger/wider mag release button. Grip texture is different too.

Don't forget the ambidextrous mag release . No need to modify the frame, but you will need to disassemble to switch sides. Not sure if new parts are needed, but in gen 3s physical modifications had to be done.