4/0, or 4over0, means that the front of a piece has the four CMYK ink colors applied to it, while the back has no ink at all. 4/1, or 4over1, means that the front of a piece has the four CMYK ink colors applied to it, while the back only has one ink color, usually black. Finally, 4/4, or 4over4, means that the front and back of a piece both have the four CMYK ink colors applied to them.