I had assumed I was getting 2160p for the 4k clips on YouTube, but when I checked in the App, 2160p wasn't even an option. Curious...
Did a little searching, and it seems the Android App itself may not provide that option, although I'm not 100% certain of this.
I did then go to the youtube website using Chrome, and accessing those clips there, I was able to select the 2160p stream. Looked awesome.
While playing, I went into my Router's UI, to see what speeds were in use. Initially when loading the clip, it went up to 100mbps (I have 200 down, 20 up), but once loaded, the speed went up and down regularly, going no higher than 25-30mbps.