I recently purchased an LG OLED55B7A. The first thing I noticed when hooking up my Shield TV was that the UI was being rendered in a lower resolution, 1080p I believe. I verified that my shield is outputting a 4K 59.940hz signal at 4:2:2 12bit Rec.2020. Can we expect Android TV UI assets to properly support 4K at sometime in the future? It is pretty sad when the UI of the WebOS native apps look 10x better than my 200 dollar dedicated '4k' streaming box.
You can force a 4k UI if you like by running
$wm size 3840x2160
via adb
The Shield probably has enough grunt to pull it off ok, but most of the inbuilt SoC's in TV's are shite
see my megathread here:
https://www.reddit.com/r/AndroidTV/comments/60eyix/underpowered_cpus_on_sony_bravia_x85dx8500d_any/