The Gen4 backstraps will make the grip small, medium or large, important if you have small or large hands. If you have medium hands the Gen 3 (or Gen4 with medium strap) might be just right. Try them out and decide the fit for yourself, either generation should function well. I have a Gen 3 G19 and Gen4 G23 and prefer the Gen4 fit and rough texture. I prefer the Gen 3 in the baby Glocks.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek