There are other considerations that I haven't seen anyone bring up yet as well, in regards to balancing a game around the option of having both a 4-man party, and a 6-man party. Just "Add more enemies" doesn't fly as well as it sounds, nor does adjust levels. Levels shouldn't really be a thing, and I generally vouch for just hiding that from the player completely. Monsters have challenge ratings in D&D, and that's a fixed rating unless homebrew is involved. That's a tool for DMs to balance encounters against party compositions. On top of that is action economy.
If you have an encounter balanced for 4 players, but want to make an alternative for 6 players, just adding two more enemies isn't a flat and evenly scaled balance. Particularly when you get to monsters with multiattack trait. Introducing two new enemies then technically means 4 more actions against the party. Whereas adding only 1 enemy in that case, slightly scews the action economy balance in favor of the player party.
TL;DR there's more manual labor balancing to be done when accounting for both options than "Just increase numbers". At least if a fun and well designed experience is desired.
So far the best suggestion I've seen is just what Fuji said, increase party size cap with warning of not being intended experience. Game will be significantly easier, inevitably so. But all that requires is a change in a handful of story goals (individual pieces of scripting that controls how parts of the game works), and... To really generalize and not write a wall of text, add two more triggers (indicators for the game to know where to place player characters in certain situations, such as dialogue, camp site for where they stand, sleep etc) and the new dialogue cinematic systems that presumably also has triggers to dictate where player 1-4 stands (hence if you mod now, sometimes two characters stand inside each other, because they both end up sharing the same trigger).
And honestly, party size discussion boils down to preference. D&D is perfectly fine with anywhere from 3-8 players. 4-6 is the sweet spot, and there's no right or wrong in that range. Though I generally don't recommend higher than 6, unless both the DM and the players are very experienced, both individually and together as a group.
Personally I think 4 is the better option for a video game. While unlikely applicable among ourselves here, one common criticism players have is that combat feels too long and a slog, that turns take too long. It was even worse in DOS2, hence both animation speed and turn change was significantly sped up and optimized in BG3. Yet it's still a common complaint among "casuals" for a lack of a better term. And if a party of 6 was an option, pretty much everyone would default to that and assume that's the right/best way to play, as mentioned by someone a few posts back. Purely pragmatically speaking, I highly suspect a higher party size would lower the overall general audience reception and positive feedback. Cooler for us more hardcore D&D players sure, but that's about it.
Some less significant cons as well is multiplayer, which already suffers with difficulty to fill a party of 4, and on a regular and consistent basis enough to see a full playthrough to its end, particularly in random groups. (Hence I never recommend it for Larian games, they're too long for most random groups to last, at least without character import/export).
