4K resolution, also called 4K high-definition (or UHD for Ultra-High Definition) is a digital TV phrase used to describe a display resolution of up to 3,840 pixels in the horizontal and a vertical resolution of 2,160 pixels. It is four times the number of pixels on a 1080p display. In cinemas, 4K resolution is slightly higher at 4,096 in the horizontal.
Recommended Reading: What is HDTV?
Standards and Upscaling
Newer standards, such as HDMI 2.0, support 4K, allowing 2160p video to be displayed at 60 frames per second. While older standards do work with 4K, there is a loss of quality and frame rate. Newer 4K displays will upconvert 1080p and lower resolution video. Do upconvert, techniques like video smoothing and noise reduction will create a picture that looks sharp on the upscaled display.
Viewing 4K Media
While visual quality is harder to detect at the higher resolutions, the picture clarity and sharpness of 4K is visually apparent on larger screen sizes (i.e. 50 inch displays and larger). Streaming video services, such as Netflix and YouTube support 4K video content and Ultra HD Blu-ray discs offer 4K movies.
Image Source: 4K UHD Picture Quality (Courtesy of Samsung)