That's up to you basically. I personally have a gen 3 but only because the 4's had just come out when I bought it, like I said I wouldn't hesitate to buy one now.
If they gen 3 grip feels good to you and you don't have a problem with the mag release, I'd save myself a few dollars an go that route. If you plan on switching the back strap or such maybe consider the 4.
Neither one is better than the other, just what you want out of it. Which ever you choose you'll be happy with.