4K is an emerging standard for resolution in digital films and computer graphics. The name comes from its approximately 4,000 pixels of horizontal resolution. The fact that it describes the horizontal resolution is contrary to the standard resolutions 720p and 1080p, which represent the number of vertical pixels. 4K represents the horizontal resolution because there are numerous aspect ratios used in film — so while the horizontal resolution stays constant, the vertical resolution depends on the video source (a.k.a. letterboxing). There are several different resolutions that qualify as 4K.
Quad Full High Definition(QFHD), at 3840 x 2160 pixels, doubles the 1080p high-definition televison standard in both the vertical and horizontal dimensions. However, it has often been debated if this resolution should be classified as 4K since it is fewer than 4000 pixels horizontally.
Standard Resolution DAR Pixels Full Aperture 4K 4096 × 3112 1.32:1 12,746,752 Academy 4K 3656 × 2664 1.37:1 9,739,584 Digital cinema 4K 4096 × 1714 2.39:1 7,020,544 Digital cinema 4K 3996 × 2160 1.85:1 8,631,360
YouTube and 4K resolution
YouTube is now supporting video resolution that's high enough to satisfy those persnickety cinematographers who shoot digital cinema? That resolution is called 4K, and at 4,096 x 3,072 pixels, it's way higher than the sharpest Blu-ray discs. But don't get too excited. Resolution isn't the only benchmark of quality, as a quick glance at these 4K files from YouTube will reveal.
First of all, you probably don't have a monitor that's capable of displaying this ultra high-resolution. This footage is captured using high-end digital cinema cameras with scary names such as Viper and Red One. Second, although the resolution might be more than 4,000 pixels wide, it's so highly compressed that you can see lots of blocky looking artifacts on board. Some clips look better than others.
As you may have guessed, this is more of a technological exercise than a practical application. It's called bragging rights, and now YouTube has them: "Now in 4K resolution!," the YouTubers might shout. Is it an improvement? Yes and no. It's the best-looking video YouTube's ever streamed, but there are still plenty of blocky blurs.
‪The Birth of a Violin‬‏ - YouTube
Click on the video above (that'll take you to YouTube's site), and select the "original" resolution for these files and you'll see for yourself why resolution isn't everything.