this is why i think four sexes is so stupid. it's just needlessly complex. nature prefers simplicity, not complexity like this.
Nature has no preference because it has no sentience. Evolution is a random process that produces lots of traits that are far from the most efficient or ideal approach to a problem; as long as they work well enough to do the job and don't impair survival, there's no selective pressure to replace them.
Besides, in an alien biosphere there could be a reason why multiple sexes provide an advantage. For instance, if a planet has stronger background radiation than Earth, it would have a higher rate of mutation, in which case it might be advantageous for the planet's organisms to have a more robust error-checking mechanism. One such method might be to have three or four sexes, meaning three or four copies of each gene, with "majority rule" deciding which genes are expressed; a mutation would thereby be neutralized unless it occurred in the genes of at least two or three of the parents, something which is more likely to be true of a beneficial trait than a harmful one.