At Vidcon 2010, Google announced that YouTube now supports video shot in 4096 x 3072, otherwise known as 4K. 4K is absolutely huge and the perfect screen size for 4K video is 25 feet. In comparison, IMAX uses two 2K projectors. On YouTube, you will need to click Original on the resolution menu instead of 4096p.
There are only a couple of videos that have 4K resolution. However, you will need to go to Youtube to watch it in 4K as embedding doesn't have this option. Watch the video below.
Here is what Youtube had to say:
Today at the VidCon 2010 conference, we announced support for videos shot in 4K (a reference resolution of 4096 x 3072), meaning that now we support original video resolution from 360p all the way up to 4096p. 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. [...] We always want videos on YouTube to be available in the highest quality possible, as creators intend. [...] 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.