my you tube resolution is 1920 x 1080 on a 4k video. Is that 4k?

[richardberndt]

How can I tell if 4k videos on you tube are actually streaming 4k

 

[Robert Ban]

If your monitors resolution is 1080p (1920x1080) then it would downscale 4k to 1080p , if your monitor is 4k then it will run 4k at fullscreen (not windowed).
Usually you can right click on video and "stats for nerds" as it is called and it should point out "854x480@30 / 854x480@30"