YouTube seems to be trying out some ultra high-def stuff with a YouTube Developers playlist of six videos picked to play in 60fps and 4K. Good luck trying to get them to play.
Here's the Star Citizen one:
You can find the full playlist here.
So far, these six videos seem to be it, you can't enable the option on your own videos for example. It's just an experiment by the looks if things, likely because 4K cameras are still crazy expensive, and most people won't be able to play back this top-spec resolution. If YouTube does roll this out for all, games might be one of the few places capable of producing a decent amount of content. At least until those camera prices come down.