All 4k displays already upscale Xbox One's 1080p output to 4K.
This is actually good news for 900p games. Most Xbox One games are not native 1080p. Currently the console upscales 900p to 1080p and then the 4K display upscales that 1080p output to 4K. Two upscaling operations blur the image slightly more.
Similarly the Xbox 360 got an HDMI model (soon after launch) and it added 1080p output (launch model was only 720p/1080i). If you had 1080p display, it was better to let Xbox 360 scale directly to 1080p. It resulted in a single frame less lag (on my TV). Most games were not native 720p (1280x672, 1152x720, etc). Upscaling them first to 720p (by console) and then to 1080p (by TV) resulted in more blurry image. 720p native games of course didn't get upscaled twice (similarly to 1080p native games on Xbox One @ 4K display).
4K output for PS4 upgrade would be much less important, since most PS4 games are already 1080p native, matching the HDMI output -> one scaling operation only.