It's like this. Whether 4 or 6, difficulty has to be based on one or the other. You can't balance the game for both unless you write in some logic that takes into account character level and party size. You just can't. If I create an encounter for 6 party members that are at Level 4 and they only go into that encounter with 4 at level 3, the encounter is going to be undoubtedly more difficult because I as a DM built the encounter based on 6 party members at Level 4. Likewise, if I build an encounter with 4 party members at Level 4 and they go into that encounter with 6 Level 5 characters, the encounter is going to be undoubtedly easier because I built it for 4. So you HAVE to take character level and party size into account when attempting to balance the game. Right now, the balance issues are because you as a player have too many choices to be able to face encounters at various levels and with various numbers of characters.
Level and Party Size versus enemy Level and Party Size is how you balance D&D. I use the CR tool for D&D 5e for all my encounters. There's a nifty app that does it for me. All my encounters work out pretty darn well when I use the app. I set the encounter Challenge Rating to Medium, I select the characters going into the fight, and boom, the app tells me how many of what type of monster I should throw at my heroes for a Medium difficulty encounter based on their level and number of party members. Regardless of 4 or 6 heroes, the app tells me how many enemies to throw at them, and the encounter works relatively well almost every time. Try it yourself. Go to Google Play Store, download 5e Companion App. If I want an encounter to be Hard or Deadly, I set the encounter level to Hard or Deadly and the app does the work for me by telling me how many of which enemy I want to throw at them that I should use. I don't even have to do the math or nothing. If this app can do it, Larian can do it in BG3 and make the battles more fun and rewarding.
I'm telling you, every time I enter one of the BG3 battles into this app, EVERY encounter is Deadly for a 4 party team. Even 6 party members often has the Deadly CR rating. It's ridiculous. THAT is why Larian has to often nerf the enemies in the game, stripping them of their WotC-given abilities to make them weaker so they can actually be defeated by a 4 party team that is way underleveled to face such encounters. I'm telling you, a single Imp in the Prologue would be more than challenging for Lae'zel at Level 1 and your Main at Level 1. Throw in a few more Players for a 4-player Multiplayer, so you have 5 party members in the Prologue, and a standard imp would become a much easier foe to kill. Therefore, in the very first battle of the game, if you have 2 PCs, Lae'zel and the Main, Larian has to make it so you'd only face like 1 Imp. If you had Us with you, maybe they could throw in 2 Imps. If you have a party of 4 players in Multi-Player, plus Lae'zel plus Us, then you could do 3 or even 4 imps in the first encounter. That would make the first encounter more fun and more true to D&D 5e, making imps actual imps and not some nerfed down version of them.
And this can be done in simple ways. They could still have 3 or 4 imps in the first area. If you move towards them after meeting Lae'zel, and you only have 2 party members, have an explosion suddenly kill all but 1 imp. If you have 3 party members (like Us is with you), have an explosion kill all but 2 imps. If you have a full party of 4 players + Lae'zel + Us, or whatever, have all 3 or 4 imps involved in the fight. You could do this same thing later in the game. Allow a party of 6 to fight at the gate when you first arrive at the Druid's Grove. With the Tieflings, Wyll, and the adventurers aiding, this battle would normally be too easy. So, the game could be programmed to spawn goblin reinforcements to come to their aid. Allow only a party of 4, the fight is a bit more challenging by itself. Maybe Larian doesn't spawn any additional enemies and just lets the player continue as is.
Facing a single owlbear with a party of 6 level 3 adventurers is likely to be too easy. Spawn an additional owlbear, the Papa. Have it come from behind, or something of that nature. Suddenly, two adult owlbears is a challenge for 6 party members. Allow only a party of 4 level 3 adventurers, then yes. 1 owlbear is probably enough for a Medium encounter.
But honestly, they could simplify all this by making it standard for the party size to be 6. Allow the players to generate up to 4 Custom Characters so that whether you play Solo or Multiplayer with 4 players, you can have 4 Custom Characters in your party giving room for at least 2 Origin Characters. Your choice, though. If you want a more challenging game, create only 1 Custom Character and try going through the game with 1. You could limit yourself to only 4 party members or less for a more challenging experience as well. Boom! There's all your difficulty settings without having to make Larian go in and create all sorts of crazy Difficulty settings to nerf or buff enemies. Balance the entire game around 6 party members so that each encounter is Medium or less with a few boss battles that are Hard or Deadly. If players want a more challenging experience, they can create a party of less than 6. And for crying out loud, use D&D 5e stats so that you have a foundation to work with!
So to summarize:
1. Easiest difficulty settings for this game would be to allow Party Size 6 (4 Custom and 2 Origin, 3 Custom and 3 Origin, 2 Custom and 4 Origin, or 1 Custom and 5 Origin). This would be considered Normal mode. Balance all encounters around this party size. Make most encounters Medium or less Challenge Rating with bosses being Hard or Deadly. Use correct D&D 5e stats for all enemies except maybe custom-bosses who might have varied stats (but again, base them on standard 5e stats. Otherwise, they aren't true members of their race. A Matriarch Phase Spider who was a magic user might be able to use Ethereal Jaunt to Misty Step across the board and then reappear as if she's teleporting from one place to another clear across a chasm, but her minions have no such abilities). Then, if players want harder fights because they feel they are too easy and not rewarding enough, they can choose to have less than 6 party members to make the game harder on themselves. Doing this would make it so Larian could focus on other, more important aspects of the game, rather than on all the crazy Difficulty Settings we've been throwing at them and telling them they need to implement. Normal Difficulty = 6 party members. Hard = 4-5 party members. Deadly = 1-3 party members. There you go. Difficulty settings resolved. No nerfing needed and no buffing just to make it work for everybody. You, the player, choose your difficulty by choosing your party size. Want the most deadly challenge setting of all? Go solo through the whole game with 1 custom character. Good luck!
2. If they aren't going to do 1, then they need to implement a Challenge Rating tool within BG3 so that the game assesses the difficulty of the encounter and adjusts the encounter to match the Party Size and Level as described above. This is the more complicated approach, if you ask me, to creating balance in the game because they'd have to do a lot of programming to add or remove enemies for every encounter in order to ensure that it was going to be a rewarding fight. Then they'd have to implement a lot of difficulty settings so that players could adjust the difficulty of the game to Easy, Normal, Hard or Deadly or whatever, or put in a ton of all these little difficulty settings that aren't really needed.
D&D 5e is supposed to be simple. That's one of the reasons WotC created it. It should be as simple as option 1 above. Want an easier encounter? Allow more PCs and allies to help the PCs. Want a deadlier encounter, restrict the party size and number of allies helping the PCs. Simple as that.