Basically Sony left it up to each developer/publisher how they want to support the more powerful hardware in the Pro. And the extra hardware in the Pro is not powerful enough to real native 4K at locked 30fps or 60fps for the most demanding graphics titles that do support 4k so compromises have to be made in various ways (less resolution upscaled or checkerboarding, FPS not locked, etc.). And 1080p users don't necessarily benefit from a better experience either. So if you care about this sort of thing you have to research each game you are thinking about getting to see whether or not it supports 4k or runs better at 1080p and what that experience might be like.
Digital Foundry is the best source I found for a technical examination of how games run on the PS4 Pro but they only examine a small subset of them: