Not fully up to Devs, itll be like PS4 Pro all over again. GPU is more than enough to handle the resolution but the CPU is the big bottleneck. Its using the same CPU from Xbox One just OC’d with some “upgrades” (Basically a PS4 Pro a PS4 jump).Everything that works on XB1 will work on XB1X.
Its up to the game developers to choose what frame rate settings they want to use. MS doesn't require 4k @ 60fps.
So games will either have to run at 4k in low FPS or a checkboard resolution with higher FPS. Sure there will be some games at 4k 60FPS but it will NOT be the standard.