Here is the article about 4k on xbox one
http://www.digitaltrends.com/gaming/...e-hard-drives/
Something to keep in mind though. There is a big chance that regardless if the next gen systems get 4k support or not it is highly unlikely to have 4k with 60 fps since there are hardly and PC hardware that can run it. On top of that fitting that much data of 4k content on a blu ray disc will be very difficult, Either we will see multiple disc, or a new way to burn data to the blu ray that takes less space