Four-color means full color, as in a color photo, that is printed on a printing press. Sometimes it is referred to as "Process Color", or sometimes just "process". CMYK are the four ink colors that are used on a printing press. In case anyone asks you, this is what CMYK stands for
• C - for Cycan, a light blue
• M - for Magenta, a kind of purplish red
• Y - for Yellow
• K - for Black. K is used to avoid confusion with Blue. And black is known as the "Key" color.
Pick up any magazine and look really, really closely at the photos and you will see a tiny "rosette" pattern made up of these four colors. No, they're not pixels! They are the four-color process that has been in use since its invention over 80 years ago. These dots are sometimes called the "line screen". It started with a photographic process and is still how all high-quality and high-speed printing is done today.