A party of six would be the best choice for a number of reasons:
It offers a much greater tactical variety to gameplay though composition. It offers a much greater social variety to gameplay, also because you can get more different people to hang out so to speak. The PC (Player Character) will be much less pigeon-holed into certain builds due to what characters you can recruit/have in party. It will alleviate much of the "miss-frustration" when you have two more potential hits.
I do not believe the added management will be detrimental in the slightest - just look at Wasteland 3! Brand new game with six people to handle and it's smooth and doesn't bog down the game in any way when comparing to DOS1 and 2.
5e adventures normally state four as a minimum number. Anyone who has played or DM'd a group of three for whatever reason can easily testify that it's borderline impossible without seriously re-balancing many things. With four, it can be pretty tight and you really need to tailor the party to a much larger extent, so much so that some players may end up not really playing what they want to play. The sweetspot is really around five to six players which allows rather diverse skillsets, maybe some doubles (which isn't at all bad, double dipping some skills in the party is Very useful).
Having the extremely low number of four with DnD ruleset is mostly a pain which cuts right into fun-time. Larian says it's easy to mod into six? Maybe they should mod it to six themselves, then? I'm very OK with waiting if it takes longer to implement. Because Larian is used to doing four character parties should not be a reason to go with four. Saying it's because of the nature of how fleshed out the characters are shouldn't be a reason either - and it IS fine if some characters are less fleshed out than others. Not every character will have a massively interesting background or super intricate plotline designed to them. For some characters, this adventure COULD be their interesting background. "Yeah, back in my younger days, I joined up with the hero who fought the Mindflayers. What a time. What a time..." *sips ale*. This offers at least one character/position of being quite fresh out the factory, so to speak, and will potentially instead be formed much more from what they experience through the game. This, to me, sounds like a very interesting take.
As a closing statement I'd say that I'll even take five character party, although I feel it's not optimal for fun purposes. Just not four. Four is simply too few.
