If you thought watching YouTube videos in 1080p resolution is mind-blowingly awesome, then the news that YouTube will also support 4K resolution, i.e., 4096 x 2304 pixels. Obviously calling 2304p doesn't quite as sexy, so that's why it's called 4K. Now, 4K has around 4.55 times more pixel count compared to 1080p, so if you have a really huge screen, this is going to be awesome. On that note, you will need a really, really, really fast connection due to the size of the video file to be even thinking of streaming the video, of which YouTube has already compiled a playlist for you to test.
youtube-global.blogspot.com - Today at the VidCon 2010 conference, we announced support for videos shot in 4K, meaning that now we support original video resolution from 360p all the way up to 4K. 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.
Full blog post here.