Well as people have already pointed out, the big reason Larian would go with four is their huge emphasis on co-op/MP play. But I personally very strongly prefer six, and believe it is possible at least on this issue to satisfy a wide range of preferences. They could make party size of four as the default for the game, and lock that in for MP, but allow players to have a party of upto six when playing SP with the understanding that the game may be unbalanced with anything over four. After all, people do like playing with party size of one as well, and that too makes for an unbalanced game, so why not allow players to go in the other direction as well and have a party size bigger than the default?
