“4K” expresses the approximate number of pixels across the width of a 4K screen. The dimensions of a 4K TV is 3840 x 2160, and for cinema is 4096 x 2160.
Making a comparison of high definition and 4K: in the space at a 1080i/p can hold a pixel, a 4K can hold almost four times as much pixels in that same space.
Below is an illustration of the difference between standard definition (720p), high definition (1280p) and 4K or Ultra HD (approximately 4000p).
4K has 8 million active pixels – 4 times as much as high definition (1080p).
High definition is expressed in less pixels than the 4K, as illustrated below. The more pixels, the greater the resolution.
The difference between the pixels of HD and 4K