Pro can run true 4K, just not very often, thus the usual checkerboarding that you see in stuff like God of War, Ratchet and Clank, etc. Looks fine but not great. The Pro is better than the base PS4, but it's underpowered compared to the X1X. The X runs almost everything in true 4K. It's really powerful, as has been noted. On multi-platform stuff, there's inevitably a clear hierarchy in quality from XBox One X being the best version by far, PS4 Pro being a noticeable step down from that, then the base PS4 being respectable but not impressive, and then the base (or S) XB1 being generally shit.
With a 4K tv and HDR, the X1X is absolutely worth it. The Pro is too, but it's underwhelming after getting used to the X.
edit: I should mention that regarding the 30 fps on X, a solid number of games on the system give you the option of 4K at 30 or 1440 at 60 fps. Pro does similar stuff at slightly lower resolutions sometimes, which is very welcome in all cases.