Strange thing is going on here. I have a 58' Ultra HD screen (3840x2160). But I have no support for 2160p - YUV420 and 2160 - RGB in the screen settings. Very confused right now...
This is my TV: http://www.philips.nl/c-p/58PUK6809...hd-led-tv-met-smart-tv-en-pixel-plus-ultra-hd
Try looking under gerneral settings and something called HDMI ultra HD deep colour.
Turn it on as it's probably off. Should work then. Did for me.