It's not that we have to play with only ONE character.
A good party build will have at least one tank and one caster anyway. And a tank/fighter is pretty important in a group and puts out most damage easily if properly skilled and supported. So no, I disagree with the one who said that having a melee fighter on your group would increase difficulty. I think the opposite is true: you should have one good melee fighter in your party.
But if you think that playing with four casters is more fun just do so. Why not? The game embraces freedom...
