Original Baldur's Gate programmer and former Dragon Age producer at BioWare Mark Darrah has a worthwhile plan for bringing the hypothetical Baldur's Gate 4 into reality. All you need to do is convince 2023's Baldur's Gate 3 developer Larian to hand over the keys to its proprietary Divinity Engine.
Darrah tells FRVR in a new interview that he imagines Baldur's Gate 4 as "an oversized expansion pack for Baldur's Gate 3," and he recommends someone "get Larian to agree to license out the Divinity Engine." But that seems unlikely to happen soon.