To boot, Larian have already said we can get mercenaries or tag along a later on that temporarily bolster the group size.
I do not believe that mercenaries were mentioned as a party booster, I think they were mentioned in the context of being able to replace companions with custom character options after already being into the game as opposed to needing to make them when you start the game. I could very well be off on this though.
Personally I want 6 for a number of reasons most of which have been mentioned. I've seen people say that a larger party would mean less interest in additional playthroughs, but I think the opposite. I'm not really a fan of most of the current companions. I'd consider an extra run or 2 maybe though if I could run a team I want and have 1-2 others tag along with out taking up primary party spots.
I plan on heavily modding the game after release and the modders have had time with the toolkit to put out finished products. That is certainly going to include mods for increasing the game's difficulty. Quite frankly the current version of the game, even without resorting to "Larian cheese" tactics, is fairly easy with the 4 player party we have. I'd be looking for increased difficulty options/mods regardless of a party cap increase.
