So just got the new YouTube update , the UI matches similar to the PS4
Problem is nothing runs in 4k. They took away the ability to choose the resolution and it's automatic based on connection.
Before the update 4k 30fps videos worked fine no problem (4k 60fps never worked on YouTube for me for some reason, yes I have hdcp 2.2)
No all 4k videos run at 1080p max and never go to 4k.
You can verify this by enabling stats for nerds.
This is a terrible update.
I'm seeing 4k@60fps on 4k content on my Shield with the new update. Very few videos are 60fps but I'm seeing it in the ones that are. What is your connection speed showing? You sure your Mi Box hasn't reset to 1080p?