+1. I've never made a video game and I'm only adequate at being a Dungeon Master IRL, so I dunno. This is my take, though.
I was fine with 4-person parties for a while. I played the s*** out of Dragon Age when I was a teenager, and although I sometimes found it restrictive because I would meet someone new that I liked, I was ultimately OK with it.
Then I played Pillars of Eternity. I felt my heart soar when a fifth person joined my party; it was like my eyes had been opened. And of course I then played BG1+2 and IWD. Still in the middle of Torment.
I totally get wanting a 4-person party for a more focused experience, but listen: if the maximum is 6, you don't have to have 6 people. If you limit yourself to 4, each char will get more XP, leveling up faster to offset the difficulty of an encounter aimed at 6 characters!
As for the multiplayer concerns, they could always restrict multiplayer to 4 and have singleplayer at 6.