4K is a display standard that includes televisions, monitors, and other video equipment that supports a horizontal resolution of roughly 4,000 pixels. The most common 4K standard is Ultra HD (or UHD), which has a resolution of 3840 x 2160 pixels (3,840 pixels wide by 2,160 pixels tall). It is exactly twice the resolution of HDTV (1920 x 1080) and has an identical 16 x 9 aspect ratio.
Tech Blog #01 