I still have yet to hear a valid argument for why they shouldn't make it 6 party members.
Here are my valid arguments for 6:
1. Multiplayer party of 4 can actually have origin characters in their party to satisfy story arc requirements like Lae'zel interrogating Zorru. Can't have that with party of 4. 2. Players have more flexibility to strategically build their party. 3. Players can literally travel with all origin characters in single player mode, thus able to trigger all story quests without having to go to camp, ask party member to leave, confirm it, walk up to other party member, ask them to join, then leave camp. Sure, more companions may be added later, but the core companions, the ones the story is being built around, can all travel with the MC together and interact, etc. without the camp hassle. 4. Lots of people want it, and those who don't can still play with only four if that's what they really want.
Have I missed any? Either way, some very valid reasons.