I played a random 1080p YT video and the player-reported data rate is at least 2.5 MB/s, peaking to 20 MB/s (cca 10 MB/s average). 4K is of course much more than that.
I just took a video as a parameter - a recent engineering animation - and got the following:
format 18 - 640x360 AVC1, V+A : ~50kB/s
format 22 - 1280x720 AVC1, V+A : ~60kB/s
format 137 - 1920x1080 AVC1, V : ~128kB/s
format 400 - 2560x1440 av01, V : ~450kB/s
format 401 - 3840x2160 av01, V : ~850kB/s
Are you sure you are not reporting mega/bits/? When you mention the «player reported data rate», are you sure that is not the "connection speed"? 10MB/s means downloading 36GB/h (one CD per minute, dozens gigabytes per hour)...
You're right it's somehow weird. Perhaps the player shows decompressed figures? Check for yourself - right-click on YT video player, show developer/debug info, and there's a chart.
I checked that interface (if you mean the "Stats for nerds"), hence I asked you if you were not by chance reading the "connection speed", which could be the closest to the values you reported and may be misleading to a glance.
