So what studios do a descent BG4 turn based D&D game?
There are quite a few who theoretically could, but literally all of those are currently busy with other games, and don't seem like they're positioned financially such that they'd actually benefit from making a game for WotC, as it were.
Obsidian is the one obvious example, but if we compare how many copies a "New Vegas 2" (I mean it wouldn't be set in New Vegas but you know what I mean) would sell compared to an Obsidian D&D effort, I think it would be a total no-brainer for them to go with a New Vegas 2 - also, Microsoft ultimately own the company who own the Fallout licence, so Microsoft could just direct Bethesda to allow that - they mostly haven't acted like that yet, but it's Microsoft - they probably will eventually.
That said, most of WotC's high command is ex-Microsoft so those connections might make an Obsidian D&D game a little more likely.
However, Obsidian have two big games on the hob right now - Avowed, out in like September, and The Outer Worlds 2, which I'm guessing we're looking more like 2026. They're presumably in pre-production on another AA or AAA game or two, which I would imagine would involve the mighty Josh Sawyer - but he's specifically rejected working with Hasbro - I imagine he'd just quit Obsidian and then be quite a catch for another studio.
Owlcat might be able to, but they'd need a lot of investment (tens and tens of millions) and have never done most of the stuff they'd need to do. They also don't have exceptional flair or much of a mainstream audience.
Paradox are basically circling the plughole at this point - they're a terrible company with low standards, who just happen to have a number of grand strategy games with good teams working on them. I will be surprised if any more good games come out of Paradox after EUV.
I will hold out hope for Supergiant Games. Which I know... never gonna happen. But still.
That's an interesting one. They have the writing talent, they have the visual flair and game design skills (far more than Larian, actually), and I feel like they're so competently run that they wouldn't have big problems upping their numbers to do a full-size AAA game. They'll probably be on Hades 2 for the next 18 months or so though. Perhaps they could go into pre-production for a BG4. I dunno how possible it would be to convince them, but it might be.
But why would Larian leave cash on the table?
Because Larian actually have principles and don't just do long-term counterproductive things just for the money. That won't last forever, but it's currently true. They're privately own - Swen has like 62%+ of the stock, so he is the sole ultimate decision-maker. And he's an eccentric (re: decent person) who values what he sees as quality and independence over $$$.
Licencing their engine to WotC or whoever is also not a "fire and forget" mission like a lot of people seem to think. They'd have to have a tech team continually supporting whatever company for probably a couple of years at least, and when that game was inevitably shameful and mediocre, some of the blame would be attached by gamers to Larian, which is just not something they want.