Ronald Held said:
Perhaps the Andorian problem is more a cultural one than biological one?
Most definitely. As demonstrated by Shar and Thriss, who went against Andorian Law and secretly consummated their bond, to the exclusion of their other two bondmates. Andorian societal practices reflect the needs for encouraging population control, as do Earth-based ones control ours (laws about homosexuality, availability of the Pill, abortion, surrogate parents, adoption, financial support to new parents - even ZPG, as in Zero Population Growth in some cultures).
Some have speculated that Earth societies' condemnation of homosexuality dates back to the survival of early community groups (ie. "populate or perish"). So, while bisexuality might have once been a legitimate option, requiring no thought as to life choices (ie. bisexuality also doubles your chance of finding a date for Saturday night), it doesn't always result in offspring, so early communities with lots of bisexuals and homosexuals didn't grow as fast, and were more easily conquered by other communities.
A study of earth snails (where the two snail partners fertilize each other's eggs), seahorses (where the eggs are passed to the male's pouch after fertilization), pandas (where the young resembles a blind, helpless foetus at birth), giraffes (where a fully-formed young can stand up mere hours after birth), crocodiles (who can change the sex of their unborn eggs by burying them at different depths in the sand), axolotyls (which can breed as "tadpoles" without ever maturing into adult salamanders), clown fish (which can change sex if too many of one gender dominate the school) and playpuses and echidnas (which are
egg-laying mammals) present a huge variety of Earth forms.
Australia recently lost a rare species: the mouth-brooding frog. Confined to areas either prone to drought, or densely populated by tadpole predators, this frog used to swallow several of its fertilized eggs - which would otherwise have dried up or been eaten - and incubates them in their bodies. Here the tadpoles hatched and grew to froglet size, when they would be "coughed up", rather than born. Bizarre - and now, extinct - because it couldn't simply couldn't compete with the further losses of its habitat.
Makes a four-gendered species sound quite believable to me.
I'm sure there are biologists tinkering around with other theories of evolution and reproduction that are more alien than any SF novel.