Whilst this topic doesn't personally concern me in either direction (I usually like to play these games solo), I see no real reason to restrict the party size to 4, other than these possible problems.
1. Cinematic Dialogues with party interjections becoming strained the more party members you have. 2. Encounters designed for one party size not scaling well in comparison to another party size. 3. Combat "feeling too tedious" when you have to handle lots of characters. 4. They want to enforce a particular game experience, which they can do by limiting the party size to 4. Lowering the maximum party size makes decisions between which classes you want to take more stringent, which might be something they want to achieve. [...] The 2nd 1 can be partially addressed by dividing XP between party members so the more party members you have, the less XP you get. This does not entirely solve the problem however, because in these types of game character's power does not scale linearly with level and so a party of 3 who is 1 level higher than a party of 6 is probably a bit stronger, especially at break points like 4 and 5. So if you were to balance party size this way, you would probably have to balance the experience around the maximum allowable party size, where reducing the number of party members would then also lead to a reduced difficulty for the player.
2.) I would vote for directly balancing around 4 party members instead of "the maximum allowable." This will ensure that parties of 6 or 2 are not too unbalanced since these party sizes are not that far off from 4. (If you directly balance around 6 party members, then a party of 2 will probably be very unbalanced)
3.) Assuming divided exp, the # of enemies in each encounter doesn't have to change. Thus, combat will actually feel faster with a larger number of players, because you're effectively taking more turns per combat round! You get to do more stuff more often (This is the same argument for why implementing pop-up reactions doesn't really slow things down)
4.) Currently, I would argue that Larian making party-class composition less important. 5e already is pretty flexible on what classes you need. Then Larian has further reduced class differences by giving out additional bonus actions, weapon actions, and the large amount of consumables. So I don't think #4 is a problem...