Personally I just assumed it's because this is Baldur's Sin: Original Gate 3 AKA they're still using the D:OS2 engine where they also had this limitation. Dagless seemed to imply there's a narrative reason for the party size though, unless I'm missreading him.
This has nothing to do with engine limitation. It’s a design choice, it’s easier to balance a game around 4 characters than 6. Same reason BioWare moved from 6 to 4.
Larian actually said they made sure that the engine supports more characters and it’s very easy to mod this, though that will trivialize the difficulty somewhat since the encounters are designed around 4 character parties.
