This might be considered insane to most people, but YouTube is now supporting 8K video resolution, also known as 4320p. This is an ultra high definition resolution of 7680 × 4320 (33.2 megapixels).
YouTube producer Marques Brownlee shared a sample of a video at 8K on Google+, linking to this video:
8K video has four times the horizontal and vertical resolution of the 1080p HDTV format, with sixteen times as many pixels overall. The number 4320 stands for 4,320 lines of vertical display resolution, while the letter p stands for progressive scan or non-interlaced.
Here is the embed of the video above:
Forum discussion at Google+.