Today at the VidCon conference, it was announced that YouTube began to support the video recorded in 4K - a resolution of 4096 × 3072. That is, now supports the original video resolution from 360p to 4096p. To realize the 4K size: the ideal screen size for 4K video is 7 meters, for example, IMAX movies are shown through 2K projectors.
Last December, 1080p (full HD) resolution was supported on YouTube. 4K size is about four times the size of 1080p. In order to watch any video in the original resolution of more than 1080p, you need to select "Original" in the drop-down menu:
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Now on YouTube there are several videos that were shot in 4K. You can see them in this playlist: youtube.com/view_play_list?p=5BF9E09ECEC8F88F . But in order to watch these videos, you need a super-fast connection. :)