this is why i think four sexes is so stupid. it's just needlessly complex. nature prefers simplicity, not complexity like this.
Hmmm, I remember "Paradigm" established that Andorian fauna is two-sexed, leaving the Andorian genome a mystery. There was a plant that matched them, but it was suggested Andorians didn't evolve naturally on Andor- or at all perhaps? Were they the result of some engineering? Given that their genome is falling apart, it seems nature is in agreement that they're not a good idea....
. Again, though, I'd like to point out my utter ignorance on the subject of genetics.