No, it's not. Modders can already have a six-men party working now, and that's without even a mere fraction of the resources, manpower and access to the game's code Larian would have in its favor. It would require a rework of the controls, on the other hand. Which it does anyway, because the current ones are the stuff nightmares are made of.
I was just watching this interview with Sven from back in September and they are aware of modders upping the party number, and he implied they would leave that to mods, saying they can offer a better experience with only 4 characters.