You are assuming that will be a BG4. And I don't think that Larian wanna become a forgotten realms studio.
Why? You become too dependent of a third party. ToB was rushed due the 3e and licensing stuff.
Im sure that Larian doesn’t want to abandon their own IP, but this is a huge break for them. They are going from a AA studio to working on their first AAA project. If they were to make 2 or 3 successful BG games it would dramatically improve and expand their resources. Plus, it does seem like they have having a genuinely good time working on this game. I will be surprised if they make BG3, it is a success, and then they just walk away without a follow up.