It is canon they marry in groups of four
Data said that Andorian marriages typically operated in groups of four, "unless...."
Actually,
this is what Data said:
Data: "Andorian marriages require groups of four people, unless-"
For all we know, the four are the bride, the groom, the priest and the stringaphone player. After all, we're talking about "marriage" here, not about sex or procreation or family life. Data is studying up on successful marriage, and the first thing up that alley is the wedding, aka the marriage ceremony.
Or, seeing how "groups" is in plural, perhaps all the wedding guests have to march into an Andorian church in random groups of four lest they defile the holy building and jinx the future of the happy couple (or trio, or quintet, or whatever)?
If Data instead is staying consistent with his previous example of marriage success depending on childbirth within the first year, then it's married life we're talking about. But again not procreation - the four required by law and custom for successful family life might be husband, wife, solicitor and godparent/politruk, with everybody else (such as children or lovers or extra spouses or co-habiting parents) being optional extra. Or then four is important
per se, for obscure numerological reasons or out of sociopolitical pressures, and if a third child is born to a couple, it has to be given away, or one of the parents has to leave the household. And until two children are born, the couple has to accommodate friends or relatives to round out the number.
All that said, my vote goes for four genders, simply because that's the most interesting angle. And never mind whether those genders are biological and necessary for procreation, or merely social and necessary for cultural reasons.
Timo Saloniemi