I am assuming this is for PVE?

If so, if it were me, I would go Paladin, Priest, 3XShaman (Trinity combo) OR Paladin and 4XShaman. These two are both proven combinations and are popular for a reason.

The only reason not to do this is if you want to play different classes solo later on or you just want variety for no other practical reason.

Why shaman? IMO it's "easier" to get more DPS out of an elemental shaman than other classes from a multiboxer point of view. Also they seem to be easier to gear up, most likely due to being able to wear chain, leather and cloth.

The tank + 4 ele shaman combo is the most interesting one, but I don't think it's relevant for you since you already have a priest.

Anyway, regardless of what classes you choose, you will always be successful with the standard trinity group.