It name derives from the 4,000 rows of pixels commonly used in Cinema. Since today's TFT displays are in a 16:9 layout and for simpler upscaling the most common resolution on a PC is 3840 x 2160 pixel - instead of 4096 x 2160 in an un-cropped Cinemascope movie (1.9:1). As of 2015 more and more television sets and PC monitors supporting an UHD resolution are available.
The next higher resolution would be 8k, effectively doubling both X and Y resolutions another time, featuring eight times the pixel count of Full HD.
The first PC graphics cards that were powerful enough to support 4k output were introduced around 2015.
PlayStation 4 Pro
Unlike the original PS4, the PlayStation 4 Pro is able to display games that were specifically enhanced for this system in 4k resolution. It was introduced on 7 September 2016.