Love YouTube HD videos? Now, it’s time to go beyond HD! And the good news is that YouTube now supports 4K videos. At 4096 x 2304 pixels, 4K resolution is over four times the size of 1080p or full HD. The ideal screen size for a 4K video is 25 feet, which makes IMAX movies look dwarfs as they are projected through two 2k resolution projectors. So, YouTube 4K videos represents the highest quality of video available today.
In order to give you a good idea of 4K resolution videos, YouTube has created a special playlist featuring 5 videos specially created by a filmmaker with access to a 4K camera. Note that 4K videos are huge in size and to view them you need a super-fast broadband connection and good hardware.
Via: YouTube Blog