I assume he's talking about Pathfinder: Kingmaker. In any case, both Pathfinder: Kingmaker, Baldur's Gate 1/2 and Baldur's Gate 3 are all CRPGs.
A party of 5 or 6 really allows characters to specialize and use specific spells, skills and feats. With 4, you need at least 1 tanker if not 2, a rogue, a cleric (or "medic" in general), there is little room for anything exotic.
I really see the difference between that and P:K, in which you can be more relaxed with the characters and experiment, it's a lot funnier.
We mustn't forget that D:OS series has a party of 4, but there is much more flexibility in the characters and they can be less specific, so what may work there doesn't necessarily work as easily in other rulesets (even if it's possible, just more demanding). D&D is the other extremity of the spectrum, even if D&D 5e allows for a little more flexibility than 3.5.
