In an article from wccftech, Tripwire Interactive has announced that Killing Floor 2 will run at 1800p on Xbox one X. The reason for this, is that the frame rate would not have been stable enough to run at a native 4k resolution.
Dave Elder, the Seinor Graphics Programmer of the game shares some additional light upon the resolution: Xbox One X was a very smooth and easy platform to develop for. It took very little engineering effort to get our base Xbox One game running on the Xbox One X. It took maybe 4 hours of programming effort total.
Killing Floor 2 runs at native 1800p, fixed resolution (no checkerboarding) on Xbox One X. We did experiment with true 4k rendering, but the frame rate drop was a bit too significant.
He also alluded that 1800p will be the optimal experience giving a perfect balance of visual fidelity and frame rate. So there you have it, thought the Xbox One X will be the definitive console version, but it seems that there are some issues getting certain games to native 4k.
Also there is no word yet on if the game will feature HDR. Regardless the game will have Ultra textures, without having to be checker boarded like its PS4 Pro counterpart.