One could argue it's exactly why a 4 members party is better, because it's more challenging. Not having the holy 6 present in your party goes for more strategic thinking as to which skills are more important than others for the next fight, making it more interesting during camp sessions for players like me who enjoyed a party of 2 in a DOS2 hardcore difficulty campaign.
Being challenging or not is all about how you balance your encounters. It has hardly anything to do with the number of "mobile parts" involved.
Anyway, the point of this thread is not to discuss if a four men party could work. We know it could. In fact ANY number could work with appropriate tuning. Not sure how many here remember that you could play BG2 (a "six-slots party" game) with any number of characters in your party, even in SOLO, because the exp gain scaling would automatically make up for any gap.
This thread about what makes it better (for me and I guess other people): the far greater variety of party composition it allows. These two extra slots are precisely what would make any party really unique.
