YouTube recently announced support for 4096p videos (4096 x 3072) which would allow users to upload huge videos to the site. However, it looks like someone had already tried upload a 4K video before YouTube officially announced it.
The video which apparently has a 4K resolution was uploaded on June 20, 2010 and based on the comments on the thread it was pretty taxing on some users systems. However, when I played the video today, it played pretty well and I could watch the video in its original resolution of 4K pixels with little or no buffering.
Also I did not see much difference between 1080p and 4096p resolution even though my laptop supports a resolution of up to 1680 x 1050. The video might play a bit slow if you choose the Original option because it is a 4096p video. Check out the 4096p video on YouTube below.
More videos in 4096p can be found on the same playlist here.