Grip is smaller without the inserts for those with smaller hands; newer models come with the beaver tail inserts; large, reversible mag release; dual-spring recoil system (some say it shoots a bit "softer"); comes with 3 mags instead of 2.
As far as shooting, there's really not that much difference.
I have both, and one feature not mentioned already is the 4 can reverse the mag release for lefties.
Still if the two I prefer my gen 3. The 4 feels slimmer with no back strap on it but thick with the other ones. But the trigger on my 3 is noticably smoother.
The extra mag alone is worth the $. Plus, yeah, being able to change backstraps on the gen 4 is nice. I personally prefer the slim option with no backstrap.