The Gen 4 has a redesigned/ more aggressive grip texture (Gen 3 is more of a smooth texture). Gen4 comes with backstraps to fit different size hands. It has an enlarge magazine release and comes with 3 magazines instead of 2 with the Gen3.
I have a Gen2, Gen 3 and Gen 4 in different models. I prefer the grip texture of the Gen4. If possible, I recommend holding them at your LGS.