I've always wondered how something like this could be tested.
The problem is, no tetrachromes are known to exist, so the experiment would naturally have to be carried out by trichromes on possible tetrachromes. The problem with this is that the trichromes wouldn't be able to see the different colors (or variations in colors, depending on how the experiment is carried out) that a tetrachrome could. Further, all current optical systems, while some are more sensitive to the human eye, are designed as trichromatic, and we wouldn't really know where to begin when designing a tetrachromatic machine (there's never been a need, and most humans couldn't even see the fourth color anyway). Therefore, the scientists (who would have to be trichromes, at least as far as anyone knows) would have no reliable way of measuring their results. So how would the scientists tell that they've found one?