I think the Steam Box will be the first console to support 4K as well.
I think consoles will not support 4K for a long time to come because they will cost too much to build, and sell, and not many people own 4K TVs except for rich people.
Steam box is a console really, so yeah that might be the first. Or sony, I think after the mishap of Xbox one launch MS may play it safe before taking on new horizons.
Though the big issue is, can downloads still be done for 4k games? Movies currently clock at 100GB.