I gotta say, my bias for growing up with them aside, I would definitely say Bionicle are just as worthy as the others for the top four. Each of those for themes did something big for Lego in their own way, and tbh they really should just do one set for each, but I guess that's not the point of this whole thing.
I do want to add to the discussion about Bionicle being trash though to say that, while I know it wasn't what everyone grew up with, and isn't one of the "classic" themes that continued to echo for decades after into cool and interesting sub-themes, I think it's inclusion in the celebration of 90 years is still important and has to do with not only the whole "saving Lego from going bankrupt" thing, but also because of the effect it's continued to have in the two decades since it started. Bionicle gave Lego the proof that they could create their own "IPs" essentially. Instead of just relying heavily on Star Wars and the like, they began making their own stories that would last more than just one theme year, which eventually led them to Ninjago, a ridiculously popular and well-received theme that I am certain is going to be looked back on in 20 years as just as important of a milestone for Lego as Bionicle was. I'll always love me some classic sets that are held together as a theme where you make the story yourself, but it is also nice to have a story to follow, with changes to look forward to each year (even if this all just serves as a ploy to sell more toys).
Anyways, at the end of this whole thing, I'll be happy with whatever wins. They all have potential to have a really nice set to honor their legacy, though I do see why Castle would be fought harder for, Lego kind of has done Castle themes dirty over the last several years. I do think the best play for them though is to just make a set for each and put it under some banner of important milestones in the 90 year history of Lego or something. Or combine all four into some giant, theme-clashing set.