Complimentary Group

[BiffWellington]

Out of those four jobs, Bard seems like the most obvious choice. Your healer and the other DPS are sort of lacking in party utility, and that's an area where the Bard really excels. Plus, it's a physical DPS job to complement the Black Mage's casting.

Edit: Pallero responded while I was typing. I'm unfamiliar with Ninja, still being fairly new to the game and all, but Pallero makes a pretty strong case for it.