Fun side note: the main reason they went for five characters rather than six was because REAL-TIME COMBAT made it messy.
I know this is another thread but I want to underline this. The BG3 engine is better set up to use 6 party members than is PoE RtwP engine.
I hoping they will change but my suspicion at this point is that the resistance to 6 party members is based in:
* 6 exposes the weaknesses of the party control system * 6 makes it clear that there are far few companions than BG2 had
Otherwise, I can't see a decent argument for 4 other than a desire to maintain the status quo.
I think one point you're overlooking is that Larian's two previous games also had a party size of four. A sense of familiarity for the decision makers, developers and the demographic they deem to be their core user base also definitely plays a role.
