4K videos are nearly four times the size of 1080p. Yes, that's huge. And yes, YouTube now supports such videos. It's a feature almost as awesome as the vuvuzela button.
This is how the size of these videos is explained on the YouTube blog:
To give some perspective on the size of 4K, the ideal screen size for a 4K video is 25 feet; IMAX movies are projected through two 2k resolution projectors.
Because 4K represents the highest quality of video available, there are a few limitations that you should be aware of. First off, video cameras that shoot in 4K aren't cheap, and projectors that show videos in 4K are typically the size of a small refrigerator. And, as we mentioned, watching these videos on YouTube will require super-fast broadband.