These numbers represent the numbers of colors on each side of your print piece. Full color printing is CMYK, which is made from 4 inks: Cyan (C), Magenta (M), Yellow (Y), and Black (B) and is represented by 4. Black & White (or grayscale) is represented by 1 (although you can have many shades of gray). A blank side or no printing is represented by 0. Thus 4/0 would signify full color printing on the front side with the backside being blank. 4/1 means full color front and black & white on the back. 4/4 means full color on both sides.
i.e.
4/0: 4-color process on 1 side and white on the other side.
4/1: 4-color process on 1 side and black/white on the other side.
4/4: 4-color on both sides
