I would consider the Definitive Edition (DoS 2) or Enhanced Edition (DoS 1) to be BG4.
Larian is not going to release this game and not add additional content and development patches. In fact, I'm betting on that the things they ran out of time to do for the initial release will be included in the Definitive Edition.
DoS 2 updates and development extended for years after release with new content, new characters, re-balancing, new items, new quests and I expect the same will happen here.
This game will be actively worked on for at least 3-4 years after release and we may get additional classes from non-PHB, just not initially.
