What do you want the g34 for? competition? range? HD? or just to have one more gun?
I have a g19 gen 4 and like it. I was thinking about getting a g34 gen 3 or 4.. at the end I will keep my g19 and my m&p which is my game pistol.
if you are into the games idpa/uspsa...and you are a very good shooter and want to shoot reloads to feel less recoil/muzzle flip, changing the recoil spring, maybe the gen3 is a little easier to do that...
if it is for range, where you wont take any advantage of the g34 then use the gen4.. there wont be a big difference. actually you can do the same with the g19, no need for a g34 actually.