I also think 5 would be good, for the same reasons Maximuuus mentioned, although 6 would be even better. I allows for so much more inter-party variety, interactions and conversations.
With the new camp supply system, you could even say that Larian could make it a player choice between having 4 party members for which it is easier to find enough food, or go for 6 people, for which you need more food. So each player could decide.