Ok, so it's a really good port so far on PS4 Pro. Some slowdowns here and there during traversal, cutscenes or combats, but it's running much better than the 360/ps3 versions anyway. The only thing that annoys me is that the framerate takes a dip when doing a blindside no matter what on 4K mode (even if you are lowering every graphical setting), which wasn't the case on 360/ps3 versions.
For best balance between performance and quality in 4K mode, turn off depth of field and camera blur, object draw distance to medium and shadows to standard or 2x. You won't get much of a hit, this won't be any worse than the last-gen version and the camera blur / depth of field looks bad anyway.
Graphics are based off the 360 version, which is a really good thing. It has proper multi-texturing on Aeos cliffs and rocks vs baked texturing, better water shader, full alpha transparency on killed enemies vs dithering based transparency, much better hair implementation (less aliased; PS3 version seemed to use a really low quality form of alpha 2 coverage), more animated shadows everywhere vs static shadows, etc.
Game is 4k60 for cutscenes, traversal and combat. but with dips. Using the 2560x1440 option provides a much smoother experience all around though if you are really nitpiccking about framerate. It also lets you maximize the graphical settings without any issue. As for me? I am using the 4K anyway since it looks much better and is still quite a leap compared to last-gen consoles framerate / graphic wise.