As someone who has been playing D&D computer games since they first appeared (alas yes I am that old!) I'd have to say I was more than a little disappointed when I tried to get a 5th member to join only to find out it wasn't possible. The vast majority of D&D games have had six party members as the standard size - Gold box series, Eye of the Beholder series, actually pretty much all the SSI games, Baldurs Gate 1 & 2, Icewind Dale 1&2, Temple of Elemental Evil, Pool of Radiance - Myth Drrannor etc etc. Even Neverwinter Nights 1 & 2 had 5 I think.
Four is just too limited for me - ideally I'd like six although I'd settle for 5. Whether they settled on four because its what they've always done or because they have concerns over the duration of combat or limitations in the engine doesn't really matter - one more character in the party would make a world of difference in allowing a bit more variety/flexibility in party composition, and I can't see that having a 5th member would require a major change in the game systems especially given that we are just at the beginning of early access.
Doesn't help that the party members you can pick up at the start are unlikable and totally opposite to the kind of party I prefer to play!