There is no SF or "short frame" Gen4 models, the SF models are all Gen3. The Gen4 features an MBS (multiple backstrap system) so the grip size can vary. The smallest, or no attachment, is the same size as the "SF" G20/21/30/29 grips.
I don't have the Gen4 G20, but do have the Gen4 G21 and it's awesome. It's also my setup for shooting .45 Super.