Another problem of committing to a set party in this game that doesn't come up in Divinity because of the way classes work is that you don't get to experience anywhere close to a wide variety of the available classes. Even ignoring the issue that a 4 person party severely limits viable party composition for a lot of players, at most you'll be able to experience 4 different playstyles and class quirks in one playthrough, which is a bummer because of the variety that the various classes offer. Plus from the point of view of someone who doesn't know D&D it's even worse because you won't get to see the full breadth of how classes play as they rise in level. Assuming act 1 ends with the characters at level 6-7, then you're just missing out on the opportunity to see how the majority of classes play at higher levels, making it harder for you to experiment and try riskier builds down the line.
