Figure 1
This differs from "posterizing" in that it reduces everything to four shades of lighting, not four colors. You could have a color map of a rainbow gradient mapped to a surface, and every soft blend of color would show up when celshaded -- its variety of colors untouched by Super Cel Shader. However, the shading would be noticeably simplified, with colors abruptly switching to lighter or darker shades in areas receiving more or less light.
For each frame, Super Cel Shader looks at how much light each pixel receives, and marks it as one of four Zones. Each pixel in a given Zone renders as though it were lit with the exact same amount of light, with the amount of light specified by the "Brightness" of that given Zone.
The Super Cel Shader's approach to "painting" affects not only the base color of a surface, but also the color maps applied to that surface. With color texture maps, then, one might recreate the star logo on Terry Bogard's jacket in Fatal Fury, or Folken's teardrop tattoo in Escaflowne, or the pattern for the magic carpet in Disney's Aladdin.
The Super Cel Shader also respects the lights in a scene, helping one blend celshaded objects with non-celshaded objects. This respect for lighting gives one a chance to blend cartooning with photorealism, in the tradition of Who Framed Roger Rabbit.
This respect for light translates into a respect for bump maps, which affect the lighting of a surface. One can use bump maps to paint shading detail on a celshaded character without unwanted Edges (which would happen with geometry details). One can use a bump map to recreate the stylized tendons on the back of Sailor Moon's hand, for example.
The Super Cel Shader's respect for light also means a respect for diffuse settings and diffuse maps, which control how much light a surface can receive. One could thus use diffuse maps to paint shadows directly onto a character. With a diffuse map, a celshaded version of Will Eisner's hero in The Spirit can always have a mysterious, mask-like shadow about his eyes.
At first glance, the Super Cel Shader interface can confuse and intimidate most people with its many "mysterious" settings. (Figure 1) For now, though, just remember this: controlling the light will control the shading.
Figure 1