YouTube has not only added 4K video but also higher frame rates, and now it's combining the two. YouTube videos that offer 4K resolution and run at 60 frames per second are now available to view online.
At the moment the YouTube videos running at 4K and 60fps are just tester samples. But this is the future for YouTube.
You can check out the videos from the one below and then by following through the playlist, although some are at 50fps since this is testing still for YouTube.
Click on the settings cog at the bottom right and choose the resolution and frame rate to see it in action. Of course if you don't have a fast enough computer, or a screen with less than 4K resolution it won't look much different in terms of quality. You will notice the frame rate difference though no matter your machine-screen combination.
For 4K you'll need a display cable that can support up to 3,840 x 2,160 resolution and, obviously, a screen that does the same.
At the moment smartphones and even most cameras can't shoot 4K at 60fps, so YouTube probably is in no rush to get the standard running just yet. It will likely be used largely for professional footage only once it becomes available as an upload option.