I use to own a Gen4 G17. It was a good pistol, shot great. I read a few reports about them malfunctioning with "weaker" 115gr ammunition, so I intentionally down-loaded some 115gr FMJs to around 1000 fps and the Gen4 G17 would still fire and cycle them just like the standard to the hotter 115gr/124gr ammo. I never had a problem with mine.
I have never shot or handled a Gen4 G19 because they are reported to be just now hitting the streets . If it has the same features as the Gen4 G17, then it will have the RTF3 texture, dual recoil spring assembly, interchangeable backstraps, SF frame, and reversible mag release with bigger button.
The dual recoil spring assembly has come under some fire since it has been introduced. It has been blamed for the malfunction problems that I mentioned above. I don't know whether it is the problem or not, I have no way of validating what I read on the internet.....because it is the internet. Just with my experience and the experience of a close friend who also owns a Gen4 G17, we haven't had any issues with the dual spring system. To me, it actually feels like it assists in reducing recoil. But with that dual recoil spring system comes a modified slide and frame, so you lose the ability to interchange change slides and frames. You also lose the ability to interchange some parts between Gen3 and Gen4 models.
I sold my Gen4 G17 to get a Gen3 G17 with the OD Green frame. I was wanting to get the Gen3 because it had the same exterior dimensions as my duty G31....and I really liked the OD frame.
My G19 RTF2 has served me well so far. It is a great pistol for CCW. As far as the G19 Gen4.....I guess we will see soon.