No 4K?

Halo 5 says it’s supposed to be 4K. But my One X won’t switch to 4K.
One X is set to 1080p 10 bit RGB because in this mode one x looks best as well as 1080p games. But 4K option in settings is ticked which let’s Forza 4 and Gears 4 go into 4K HDR fine.
I switched the console to 4K but it didn’t bring any visual improvements.
Do i need some kind update?

H5 is enhanced for X, but does not play at native 4K 100% of the time (it uses dynamic resolution) nor does it output in HDR.

For more details, read this article by Eurogamer.

It may be your TV. My TV plays the game in 4k fine. When I turn on game mode it won’t play in 4k because of the higher input lag. I have to turn my one x down to 1080p to use game mode on the TV with less input lag.