As someone that has already gone on record as wanting a party of 5 or 6, I'd be completely fine if they kept the game balanced around a party of 4.

With a party size mod, I'm assuming that the game already wouldn't be balanced for what I was doing since I don't see modders adding in enemies. But, I'd have a far easier time getting to play how I wanted without having to toy with files and wait for people far smarter than me coming up with fixes if a patch breaks the mod. Large red font at the start of the game saying "Hey, loser, you're gonna make the game super easy for yourself. Proceed at your own risk!" would be enough for me. I'm claiming the responsibility for my actions & choices at that point as I've been thoroughly warned.

If they're worried about people using larger parties for an easier time getting the achievement of beating the game on the hardest difficulty (if they do that), they can keep it specifically for completing it with parties of 4. But then, I've never been an achievement hunter so your mileage may vary.