I also like the party size of 4. Im reminded now of xcom where i always have more fun early on before i expand the squad size. 4 feels tactical. When you throw in summons even having a single character who can summon brings you up to 5, which feels ok. Now imagine a party of 6 where 2 or 3 characters have summons and suddenly its all messed up. Some people will say "well just dont do that :^)", but what they are effectively saying then under those circumstances is to not use entire classess who would be enjoyable in a smaller party limit but become ponderous in a larger party limit. They will also say "just use fewer people", this would be the equivalent of telling people to simply ignore bad balance. Ultimately the normal way you play a game is with the players guided by the standard mechanics and rules. The normal way to play a game is the expected way to play it, and saying its just fine for the expected way to be something disrupted that you then have to play around is simply asking for a bad game design.
