4/0
4/0 is a trade designation for a printed piece with four colors (full color) printing on the front side and no printing on the back side.

4/1
4/1 is a trade designation for a printed piece with four colors (full color) printing on the front side and one color printing on the back side.

4/4
4/4 is a trade designation for a printed piece with four colors (full color) on the front side and four colors (full color) printing on the back side.