I can't feel or measure any real difference in grip size between the SF and Gen4 grips.
That said, the reasons I would go for the Gen4 are: the mag catch is easier to reach/use, the recoil spring assembly is improved and appears to help handle recoil a bit better, and the grip surface works better for me....less 'slippery'. Another some people would go for the Gen4 is that the grip can be made bigger.
U.S. Army-MNNG 73-83 95 Bravo
NRA Life Member