A lot of the games on Xbox One X will not run at native 4K. Some games like fighting titles eg. Killer Instinct, or racing games like Forza will, but that's mostly because those genres are less graphically demanding than others. Like the PS4 Pro a lot of newer games will be running somewhere in the 1440p-1800p range and then using upscaling to get to 4K, or they will be using checkerboard rendering, where only half the horizontal resolution is rendered and a predictive algorithm is used to fill in the unrendered pixels to reduce GPU load.
Truly native 4K and 60FPS is still quite difficult to achieve in the most graphically demanding titles, and you won't see that on Xbox One X, those games will be running at a sub 4K resolution and...