What do you mean?
Just because a computer can handle 4k, it doesn't mean the display is. The video is downscaled to the resolution it's playing at (aka video size) anyways.
Go into where the display settings are on a mac and look at the resolution. It should look something like this:

4k is 3840 X 2160 pixels.
EDIT: Im an idiot. I actually clicked the YT link and damn that is a crisp video.