4 shaman at 60, what for a 5th?

[Crucial]

I'd have to say Druid. Boomkin + Shammies = Crazy DPS. My 4x shammies are at 70, and swapping one out for my currently 62 druid nets MORE dps than 4x70.

Imp Boomkin = 3% spell haste
Earth and Moom = 13% more spelldamage
Imp Faerie Fire = 3% spell hit and 3% crit (doesn't stack with ToW)
Insect Swarm = -3% hit for mobs

then who tanks?

Aside from elementals, you don't need a tank... even for heroics as long dps is high enough.