I think it fundamentally comes down to whether your sense anomaly represents a significant disability. Colour blindness is a disability, but not one that's significant for the vast majority of people who suffer from it - I've worked with multiple colour blind graphic designers & they were good at their job. There's very little impetus to even seek diagnosis - if they weren't working in a colour-focused industry I suspect they may not have ever realised they had a disability at all.

Tetrachromacy then is an even harder case because it's not a disability at all. The impetus to seek "diagnosis" is zero. Also, even though as you mention there's technically multiple ways of detecting the various factors that need to coexist in tetrachromacy (i.e. (1) sensory testing, (2) physical presence of extra primaries, (3) neurological processing pathways), the latter two are either not directly detectable or never directly tested for - even in speculative cases of people having a 4th primary, the number of primaries present is generally hypothesised via some other avenue like testing for anomalous trichromacy. Ultimately we're heavily relying on direct sensory testing which is almost nonexistent in the general population. There's no way to accurately speculate on how prevalent it might be.