Sure there is a problem with using RtwP examples but TOEE did just great with larger parties. TOEE had lots of problems but combat wasn't one of them.
Get onto the Solasta forums right now and you will find people discussing how to build characters to replace the thief -- do you build the ranger into the lockpicker / trap disabler or is better to give that to the greenmage? It's a problem I'd rather not deal with. I liked the BG2 party -- lockpicker, meat shield, healer, archer, mage, wildcard.
In a 4 person party I just don't have space for wildcard (sorry bard) and I don't like the building against class strengths -- yeah you can make the bard the trap disabler or have the mage pick locks but that's just not as much fun as having a thief in the party.
This isn't Solasta, if I wanted to play Solasta, I would be playing Solasta. That comparison is just getting so old and used, it is practically Pamela Anderson. Every game has their interpretation on party size. The most common in ALL of them is 4.