If the Gen3 guns fit you then I would not do it, they don't have the Gen4 9mm's squared away yet, it's a fact, plain and simple.
The new grip and mag release are awesome, great improvements, that said, the new recoil assemblies and trigger bar changes have eroded their reputation for reliability to the point where they're really not features but potential problems.
I put 6000rds through one Gen4 G17 waiting for it to work right and finally just got rid of it after all it's problems and went back to the Gen3 and experienced no-problems just as expected, a few months later I decided to give it another chance and picked up another one, sold it about 700rds later, was still choking and still wasn't right. I never felt confident enough in either of them to carry them, actually had a few malfs during IDPA matches that hurt my scores pretty good... and I'm not the only one either, quite a few guys I shoot IDPA with went to the Gen4's after they were released and to a man everyone is back to the old reliable Gen3 models.
The Gen3 9mm guns run like clocks, get one of those if you just are after a great gun that's a modern classic.