It is an interesting (even if disappointing) design to go with a 4-person party, but it seems to tie to two key things that might explain Larian's reasoning. And there are pros and cons to both of those elements.
1) There is going to be a story point where we *permanently* lose the companions not in our active party. Thus, it comes down to picking which character's personal questline sounds most appealing to you for that playthrough.
- The UPSIDE is this adds a bit of flavor to roleplaying. My pragamtic and rather shady half-elf rouge does not care at all about Gale and Shadowheart's respective goddess obsession, so she will be leaving them behind to hunt for either a cure with La'zael or a chance to control the tadpole with Asterion. Wyll gets to come along too on that adventure, because he is no stranger to the darkness despite his good boy act. On the other hand, my elven druid is intrigued by Shadowheart's mysterious past, and is sympathetic to Gale's predicament. She is also fascinated by Lazeal's culture. She also thinks both Atserion and Wyll are abominations that disrupt the balance. New companions could cause more iterations with whatever type of char I build. Etc ad nauseam.
The DOWNSIDE is that many players want the chance to experience the full game (or at least all the companion side-quests) in a single playthrough, and have no desire to fire it up again and again. Plus, to get all of their banter, one would have to play through many times to get different party events. It is already annoying to miss so much when half of your companions are stuck at camp, let alone when they are totally inaccessible somewhere midgame. Definitely sucks. Also -- just as a side note to this --- I really hate the idea that a story event separates my party --- I would much prefer that my choices make some of them leave in frustration/anger/resignation. That would make much more palatable and perhaps even exciting. But that is for another topic.
2) From what I understand from reading other threads, the 5e ruleset coupled with the "extras" (what some call cheese) that comprise the core gameplay means that you really don't have to build your party with all that much thought.
- The UPSIDE is that you don't have to have a rouge/tank/catser/healer set-up anymore to win fights. From the EA, it seems pretty clear that you can take whoever you like in your group out adventuring, and you will probably be OK. So there is actually a bit more freedom in party builds because the game is quite forgiving, and thus a 4 person party is not all that problematic.
- The DOWNSIDE is that this is less challenging gameplay overall, and would likely be more fun with a 6-party build and more stringent DnD rules (although I am talking out of my nose on this one...but I imagine it would work that way).
In summary---- both justifications for having 4 vs 6 person party have pros and cons. We could have a 6-party build but would need more challenging gameplay to balance it, and we would end up doing companion quests that are likely out of character from a role playing perspective (at least for some roles). OR, we could have a 4 party build which means missing out on content on a single playthrough, but having a more relaxed gameplay experience that does not overly punish us for our team choices.
Anyway -- sorry if this has already been stated by others. Did not read all the posts yet! Just trying to sort through my own thoughts about it more than anything. But on the modding note --- I think these two key points are important to consider for whatever mods are even possible. If a mod expands the game to 6, we definitely need some upping of the difficulty as well (ie, beyong more enemies or more sponges). AND, it might break the game's narrative structure, from what I understand from the inevitable party split that is coming.