SInce the starting point for the game's design decisions is obviously co-op play, it will be 4 because 6 is too much for co-op play. People like me who would only play single-player and as such would want 6 are not going to matter in Larian's calculations.
Its true that alot of people play solo. Even if I've played co-op several times I'm pretty sure I've clocked more hours solo.
I guess its a matter of prioritization or if they can automate the scaling somehow.