Nice pistol and good choice.
Nothing wrong w the 3 at all, it's won a lot of trophies.
Gen 4 you get texturing, dual spring, and multiple back straps to help w your grip.
Being a Glock it'll digest about anything you feed it. Mine seems to shoot 124 gr a little better than the 115s if that matters.
If it's all about price, buy whichever one is cheaper. If it's a one and only pistol and you like the "new thing" (nothing wrong w that btw)then get the Gen 4.
Point being, they're Glocks. I'm kind of a Glock guy so being a Glock says it all for me.
Either will be fine.