Up to 1440p... If you want up to the 4K, you should get the XSEX, this never confused me... Only thing confusing was the name when the XBONEX was still around..
Now it's super clear, Games will be made for the S (target up to1080p60 now apparently) rendered and upscaled up to 4k on the X.
IMO base should have been XSEX, but, i do get where MS is coming from with this S model.