You can only use 3 characters in battle or 4 if you have guest ((SPOILER)Wossler, Larsa).

I have already played the game and my team was:

Vaan - Specialized in everything
Ashe - Specialized in everything
Basch - Specialized in everything

(SPOILER)Three of them have high level as well as high stats and there's no use giving them job, it's better to teach them all the skill since this is like Sphere Grid, you must learn the skill in order to move, the skill is mixed so no matter what, your character will still learn all types of magic

Sometimes, I switched around with Fran, Balthier and Penelo. (SPOILER)Three of my weakest character

boys from the dwarf, (SPOILER)You can choose to not use Vaan.