Whoever picks this shit up is gonna have a hell of a time. Baldur's Gate 3 is a clear evolution of Larians own engine and style. The UI elements all feel like Divinity to the point if it was called Original Sin 3 no one would bat an eye.
The only way I can see a sequel being successful is if whoever gets it works out a deal with Larian to use the engine and get support in some way. In a sense this would be kinda hilarious reversal of WotC getting a fee/royalties from BG3 to Larian getting it for BG4.
I'm not sure any of the big sorta CRPG devs out there would fit the bill. Owlcat, Obsidian, Bioware, Harebrained, Inexile etc they have their strengths and weaknesses. None of them really have the expertise in voice acting like BG3 clearly showed, Owlcat works well with their systems but they are primarily isometric rpgs which BG4 will never be, Bioware psedu action rpg style and love of spectacle sucks and probably not a great fit, Obsidian I don't think has the scripting chops for it anymore etc. If Biowares output was more like the Dragon Age Origins days I would absolutely say them, as BG3 felt very similar in a lot of ways.
Personally...I think I would want someone who I had faith in writing good characters, can write with wit and script faithful to the source material and just have them license the engine from Larian and build a scenario around that. My mind kinda goes to Edios-Montreal after their work on Guardians of the Galaxy, which was also done with massive respect for the source material and performances were fantastic. I wonder how they could do if they were just given Larians tools and asked to make the game.