Correction, I know that fan fiction has Andorians with four genders because I written some. Even worked out exactly who does what to whom, and in what order. I seek out Andorian fictions. When the gender is part of the story, which isn't always, the number is usually four. Although I have read sories with just two genders. Writers choice of course.
But what dates are you looking at here? AFAIK, the four-sexes angle derived from the pro
novels, commencing with the "DS9: Avatar" duology, and based on Data's brief quote about Andorian weddings in "Data's Day". The Lesiey Fish material dates back to the mid 70s
. Many of her cohort of fanfic writers used her three genders concept over many years, and the Pocket folk deliberately avoided reading any of that "fanon" material to ensure they weren't overly influenced.
use a four-sexed paradigm in your fanfic, did you conceive of it independently, or did you extrapolate from the discussions generated by the pro fiction's take on Andorian reproduction?
By the way, to get back to the original question, an early Andorian hybrid from the fanfic turned pro fiction:
Andorian Doctor Shona Exar was a female biologist with twin sons in the short story Ni Var
by Claire Gabriel, Star Trek: The New Voyages 1,
The boys' physiologies reflected their parents' mixed heritage; Shona's husband, Albar, was a rectangular-headed, red-skinned Fornaxian. The boys had purple skin, black hair and Andorian antennae, but they were complete opposites in personality. It was revealed in the story that the scientists had produced a single hybrid child by natural means. Internal incompatibilities made it necessary to "twin" him with experimental 23rd century technology. Someone then tries to use the technology on Spock. (It reminds me of the B'Elanna Torres episode of VOY where the Vidiians split her into a human and a Klingon, but it dates back to the 70s.)