At VidCon 2010 conference, Google said that "YouTube now support 4K videos (i.e. original video resolution 4096 x 3072). The ideal screen size for a 4096p video is 25 feet; IMAX movies are projected through two 2k resolution projectors. To view any video in a source resolution greater than 1080p, select "Original" in the video quality pulldown menu:
Check out the 4K videos using the link below; each video is created by a filmmaker with access to 4K camera," stated Google.
Because 4K represents highest quality of videos, there're a few limitations you should be aware of; "First, video cameras that shoot in 4K aren't cheap, and projectors that show 4K are typically the size of a small refrigerator. And, watching these videos on YouTube will require super-fast broadband."
More Info: 4K Playlist