Handprint's articles on color are by far the best resource I've found on the Web for explaining color vision. (If you think you have some clue about human color vision, or know what "primary colors" are, you're probably quite mistaken. There are a lot of very interesting details.)
This article on color vision in blue-code monochromats discusses SIGFPE's slightly ill-stated conjecture. Specifically, it's true that our "rods" have a different color sensitivity than any of the three kinds of "cones". In theory, that makes us all tetrachromats, except that during the "photopic" conditions (ordinary fairly bright light) when our cones work well, the rods shut down; conversely, under "scotopic" conditions (dim light, "night vision") when the rods are active, the cones don't have enough light to function. There is a narrow "mesopic" range in between, when both rods and cones are active, but even then it appears that the rods simply reuse some of the cones' visual circuitry, so we're still trichromatic (though our color sense tends to skew because of the way the rods' input is mixed in; specifically, everything tends to look blue). The first link above (from Handprint) talks in more detail about the stages of color processing and how inputs get "mixed".
All in all, the important part is not that the hypothesized tetrachromatic women have four kinds of cones, but that they may have the neural circuitry to make sense of all that input. The psychology of color is a complex and fascinating subject; I wonder how they see the world (if they do indeed exist).