As for Andorian mating, someone did an analysis over on the Trek Lit forum a while ago and showed that four-sexes reproduction would create huge problems - because 2 out of those 4 sexes would be far more common than the other 2. (Say, if we represent zhen as XXXX, shen as XXXY, chan as XXYY and thaan as XYYY, and then list all possible combinations, XXXY and XXYY would be more than 4 times more probable than XXXX and XXXY. Or to be precise, the ratio would be 13:3.) In that light, marriages in four for the purpose of reproduction would not make sense.
I'd like to see that discussion. Any clue what it was called?
Because I can already, off the top of my head, think of a way to do 4 sexes that doesn't have that problem:
XX XY XZ and XW. When the dna from the gametes comes together to form the zygote, there is a little competition of some kind, and each chromosome is individually adopted from one parent only. Thus, it might even be possible for an XZ child to have gotten his X from his XW parent.
I'm not a big fan of the 4 gendered idea. It seemed a bit contrived to me.