Definition of "quadtone"

In Photoshop’s Duotone mode you can create monotones, duotones, tritones, and quadtones—grayscale images to which you add one, two, three, or four colors.

1995, Adele Droblas Greenberg, Seth Greenberg, “Converting from One Mode to Another”, in Fundamental Photoshop, 2nd edition, Osborne McGraw-Hill, page 380

When using tritones or quadtones (in which three colours are blended together) all the secondary colours are mixed together.

2003, Duncan Evans, “Duotones and Tritones”, in A Comprehensive Guide to Digital Portrait Photography, AVA Publishing SA, page 115

The two colours need not be black and grey; computer duotones can be created from a greyscale original in any two colours; tritones use three inks and quadtones four.

2010, David Präkel, “Duotone”, in The Visual Dictionary of Photography, AVA Publishing SA, page 97