I got to see PS4 Pro in action the other day.
They had Horizon: Zero Dawn running on a huge 4K HDR screen.
This isn't a native 4K game, but damn if I could tell there was much difference. The IQ, even on such a large display when getting fairly close to it(just a few feet) was amazingly clean. The whole image was just ultra crisp and cohesive.
They also demonstrated it with HDR off and on. It was also quite impressive. I didn't notice much difference to the darker areas or any real improvements in color production, but the brighter areas(largely just the sky and the god rays and whatnot) certainly got a lot brighter and added a higher, more natural contrast to the scene. It was almost uncomfortably bright, but in a realistic way, as you'd expect were you pointing your head at the bright sun.
I get the feeling HDR implementation in games, at least early on, will probably be limited to certain enhancements rather than a ground-up remastering to fully take advantage of HDR. I've heard improvements in games like Deus Ex and Forza Horizon 3 saw similar limited improvements.
Anyways, yea, if you've got a 4K HDR TV, I'd say you owe it to yourself after spending that money to get a PS4 Pro. It's extremely impressive and probably the nicest I've ever seen a game look in my life. Even if you've just got a 4K TV/monitor, I think it will be well worth it if you're interested in the PS4 library. I'm still strongly getting one even just for my 1080p monitor. IQ improvements should still be extremely good.