Back in November 2009, Google [GOOG] announced that 1080p full HD video was coming to YouTube. That of course happened and we now have access to 1080p when users upload video at that resolution.
The next step up from there is 4K video that has a resolution of 4096 x 2304 pixels making it four times larger than 1080p.
4K video has an ideal screen size of 25 feet. Google point out that IMAX movies are projected by two 2K resolution projectors.
For those wanting to see how 4K looks on YouTube you can check out this playlist which has a number of 4K video samples for you to view.
A few things to note here are that your screen on a computer probably wont handle that many pixels, so you wont get the full effect. Also, with 4K video being a lot larger than regular YouTube videos you might not be able to stream it as smoothly as you like so you might be best letting it download most of the video first before watching it all.
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.