Alright so I'm a veteran D&D player of 15+ years across each of the editions. I am also a huge fan of the old Baldur's Gate games (I still play them today). I get that many video games have started going with the 4 character party cap, and some people are ok with that, but overall I believe it severely reduces the experience, it destroys replayability, and will inevitably only serve to bog down gameplay. here's why:
1. Baldur's gate (as well as all the classic D&D rpg titles: Neverwinter Nights, Icewind Dale, Planescape: Torment, ect) have always gone with 6 characters in a party and there are good reasons for this, but for Larian studios the best reason to consider it is because reducing the party size to 4 will make the game feel less like a Baldur's Gate game, and more like a Divinity game. which don't get me wrong Divinity is a great series, but in the end it's not Baldur's Gate, and that's what fans who have been waiting years for this really want. I have already seen a lot of reviews and comments about how BG3 doesn't really feel like baldur's gate, and I think this would go a LONG way to solving that.
2. Having a party size of 6 means the player is not as "locked in" with their character selection. In a game where I can only have 4 characters at a time I am inevitably forced to make my selection based on the bog standard "tank/dps/healer/buffer" type of party, because without those key roles being filled the party becomes unbalanced. you have a little bit of wiggle room if you specialize those 4 characters the right way, but not much. and inevitably some characters are simply going to get left behind this way. In my playthrough for example, I really like Wyll's personality, and his combat ability is great, but I rarely have him in the party because for me him and Astarion compete for the dps role, except astarion can pick locks much better than Wyll can, so it's a no brainer that I need Astarion. For every player that might be a different case but with 4 character max there will always be a character the player wishes they could bring but can't because their playstyle is in competition with another character they think they need. In a party of 6 however the player has certain options, not only can he fill those roles more easily, but he can double down on one or two, which allows for a huge variety in the types of parties available and therefore: endless replayability. It allows players to bring along 2 extra characters for no other reason than the player knows that one or even both of those characters will be useful or relevant in the upcoming storyline archs, AND, it allows for the main character to be made in ways that are "outside the box". For example, a player might want to make his main character be more of a faceman, with all his specialization focused on utility spells and abilities that help him outside of combat, while leaving the combat skills to the rest of the party. Most people won't be able to do that effectively in a 4 character party.
3. In the end, the way the game plays right now with a 4 player party, I found that the lack of 2 extra characters just meant that I was constantly having to return to camp for no other reason than to change up my party to have the right characters for the moments I wanted them in. And that just became annoying really really fast. It would be even more annoying if I was unable to go to camp at any given time I wanted without consequence (which is something that in fact NEEDS to be addressed too, resting is too easy). And so that's really the problem I see here, is that a party of 4 has no actual benefit to anything because you can change which characters you have at any time anyway, and instead it's only going to result in the player getting annoyed because they have to return to camp twice every time they encounter a locked chest, once to get the rogue in the party, and another to get their chosen character back (or what ever example you want to think of, wizard for detect thoughts, fighter to move a rock, etc).
4. A party size of 4 means no movement formations, which isn't actually a huge deal, but it's happened enough times where I'm trying to get my main to the front of the party but one of my companions ends up triggering the event instead just because they don't know to stay behind him. And again, having this feature would make the game FEEL more like a baldur's gate game.
Simply put, 4 characters is not enough for a D&D game. A tabletop game yes, because there are other problems with having 6 players at a table, like how distracted everyone might be. but for a videogame this is not the case in fact it's the opposite. the single players will want more out of their party, so that they can explore the game more fluidly without breaking every other moment to change their roster, and a multiplayer group will probably enjoy having more slots for players to join in. This is why I think Larian should reconsider the 4 character party cap.