"4K" is the term used to describe images that have approximately 4K vertical lines. In digital cinema, a 4K image is delivered to the theatre in a digital "container" 4096 pixels wide by 2160 pixels high. A 4K image with a 2.39:1 ("scope") aspect ratio could be supplied having 4096 x 1714 pixels. A 4K image with a 1.85:1 ("flat") aspect ratio could be supplied having 3996 x 2160 pixels.

"2K" is the term used to describe images that have approximately 2K vertical lines. In digital cinema, a 2K image is delivered to the theatre in a digital "container" 2048 pixels wide by 1080 pixels high. A 2K image with a 2.39:1 ("scope") aspect ratio could be supplied having 2048 x 858 pixels. A 2K image with a 1.85:1 ("flat") aspect ratio could be supplied having 1998 x 1080 pixels. While 2K is the commonly used projection format today, most if not all so-called 2K productions are made to fit in HD (high-definition) pixel arrays, which are 1920 pixels wide.

HD content is very close to 2K and is widely used in Cinema production. (Superman 2 etc)