I don't mind a 4 person party. It's not the worst thing in the world. But 5 would be far more comfortable and also make for more diverse party compositions.
I'm going to make a comparison to MMO's. Completely different genre, I know and that might not be fair. But the Problem is very similar and relates. Particularly compared to FFXIV Online. But it is a problem many "new" (post WoW) MMO's have.
That is called the Trinity. Let's say you have a raid, you have 2 tanks, 2 healers, 4 DPS. There are no utility jobs. Or if there are, they are niche and not desired to be in the party. But in most cases with MMO's nowadays, again, using FFXIV as an example, a Bard is considered a DPS. A red mages is a DPS, and so on. It's a set group of those 3 kinds of jobs. No room for a true support job. Those classes may have support abilities integrated into them, but they're still a DPS. Certain jobs are preferred over others. Because there is no room for a class that doesn't offer the maximum amount of optimal output.
Final Fantasy XI Online did it extremely well. A standard party is 6. A tank, mage, healer, 2 DPS, support job. In that game there were several support jobs. You were encouraged to experiment. Much of the content in that game required support based classes.
Granted BGIII isn't a MMO but I feel a similar can occur. Instead of a Trinity we have a Rectology? lol. Or Rectangle based system. By having only 4 members, you limit the ability to designate someone to a utility/support job. Granted we can still have a varied composition of members, but many people are presumably less likely to experiment with classes. The average player will probably go Warrior/Rogue/Damage Dealer x2. Otherwise they might think their damage output is too low. Or survivability. Adding even 1 extra slot to party members creates a possibility of so much more diversity and party experimentation. I hope 5 members at the very least get considered.