Sorry if I missed it but is this now supported or am I experiencing some sort of glitch? I rebooted my shield today and when it came back on everything was small. I decided to switch the resolution to 1080 and everything looked normal. Flipped back to 4k and its small again and I am able to see many more apps and rows which to me is what the UI would look like in 4k. I tried searching for news about it but I just found older posts about how it isn't natively supported yet.
Probably a bug, Google won't care about 4K support until more phones other than Sony come with 4K displays.
Sadly no matter what they claim, Android TV is only an afterthought for Google.