I like the number 4. And I think the game is clearly being balanced for 4 player party sizes, so that's a major point (and the whole "party formation" part of the game becomes obsolete in single player if they don't add more friendly characters with a 6 player party size). I think they could workaround the combat/gameplay advantages by maybe adding a modifier to groups larger than 4. Like -1 on all rolls for each party member above 4. Skill checks become tougher, magic missiles becomes trash tier in a 6-man.
Additionally, I don't really think 4 party members is all that restrictive. You can pretty much cover all skills and proficiencies with just 4 characters.
Lastly, we only have half of the classes so far. And with bard, druid and paladin all coming in the final release, I think there is going to be plenty of healing power, making the "healing slot" almost trivial... heck you could have 2 healers, maybe 3 healers... they're all gonna be able to fight anyways.
This is a role playing game. Variety in parties is the point. Cover all skills and proficiency is not.
