Unlike DOS- where you could make everyone a hybrid of everything and just add Hydro and Aero on half the party and the other half have Geo, Scoundrel and Warfare - this is Dungeons and Dragons and each class is limited on what they can do and even tough some spells might repeat for a few classes, their class features are unique and only them can bring it to the table.
This means that it's hella restricting only having 4 because you'll miss so much on those jack of all trades classes that cannot be included since you need to min max and follow the basic model of 1 Melee, 1 Healer, 1 Rogue, 1 Magic Caster. It's not diffcult at all to understand that if you have a party of 5 or 6 you can add more versatility and enjoyabilty in the RPG aspect so you can play whatever the hell you want even if they are not the best options/selection.
4 people parties are doomed to either be min maxed so they can take the whole game or forced to multiclass in the future.
