I had a quick thought about multiplayer too. I wonder if the ultimate reason for the 4 person party may be due to multiplayer concerns? Trying to organize sessions around 6 people is a lot more complicated than 4, and it also means each individual player has to wait longer before their turn comes around. However, there is another argument to be made that a proper reaction system will keep everyone much more engaged outside of their turns.
At the same time, increasing the party limit to 6 would also be a huge boon to 3 player parties. How? Because each player will get to control one companion of their choice as well. And a 2 player party will get 3 characters to control. 4 party limit multiplayer is kind of in an awkward state where the overall experience is balanced around 2 or 4 players, because 3 players means one person will get to pilot a second character while the rest only get their primary character. Granted, increasing the limit to 6 pushes that awkwardness to 4-5 players, but it's also much easier to organize 2-3 player sessions as opposed to 4 and beyond.
A 6 party limit would also greatly enhance the origin system for multiplayer purposes, I'd think. I still think the system is a waste of development resources, but if all 6 players decide to play as origin characters, it'd greatly enhance the experience of using that system in theory, as all six players would get to experience that unique perspective together. The room for more companions is also a bigger boon to the experience of 2-3 player parties as well.
(Also, a cynical analysis would be to think that Larian doesn't really consider the inventory management to be that messy because it's inherently less of a problem in multiplayer.)