Do be aware that it looks like it's an area that needs further study (and apparently the "fourth" cell may detect light between red and green). On average, humans are (generally) not tetrachromats. Poking around on a cursory search suggests that only one subject in that 2012 study expressed attributes that were definitive evidence of the existence of tetrachromacy in humans. Apparently there's some ongoing discussion as to whether or not the optic nerve is capable of funnelling an extra color channel to the brain, so that'll be of interest to follow.
Importantly, I was thinking along the lines of some insects and spiders that are capable of seeing into the UV spectrum for sexual signalling or locating food sources (e.g. flowers). Which reminds me that there's been some evidence that suggests those who've had cataract surgery may be able to see deeper into the violet spectrum without a UV-filtering obstacle in place. But that's likely just the function of the blue-sensitive cells.
