Do you know if the show was originally planned for two or four seasons? Idk, but the show gives me the impression that Nefcy's original concept finished with BfM.
Hard to tell, since Disney greenlights seasons one by one, but as far as I know the renewal for S3 came very early, when S2 was still far from being done, while the renewal for S4 came a bit later (or at leas that’s the impression I got from the comments of some storyboarders). Since S1 introduced Heinous and the Blood Moon so early to then barely address them in S2 I think a third one was planned if not from the beginning from very early in production. It’s possible that they then adapted it once a fourth one was approved, taking more time to explore the characters and the lore, or changing some things around. Would be interesting to get an official answer about things like this, to get to know how the overarching writing for a show like this works!