ForceUser
Explorer
I love adding on classes whose strengths are group support so I chose bard. But as others have said, any class works well as a 5th. In a recently begun campaign with the fab four config I chose monk, actually, because I rolled amazing stats, found the monk role appealing, and figured I'd give it a spin. I might have gone bard had the DM not made it a prestige class (boo, hiss). There is no wrong choice for the 5th guy when all the bases are covered.