I thought it referenced those four colours (which is why "4-color" T-Shirt printing is so expensive), but didn't know it was also in reference to comic-book printing.
Just to clarify, it's any kind of printing, not just comic-book printing. A magazine that prints in color is using the four-color process (unless it's just black-and-white plus one other color, which is referred to as "two-color"). Same with newspapers.