Why four toons and not five?

Prot Pallies are superb tanks, a strong option and easy to macro.
Disc Priests are great healers too.

Not sure how shadow stacks up now, although it was supposed to be buffed in 3.3.

If this is a pvp composition, I'd ditch the Paladin for another caster.
If it's a pve group, I'd probably exchange the two shadow priests for say a warlock and an elemental shaman.

Not saying don't go with priests, if that's what you want to run...