I run this exact team with a pally:

pally prot
shaman elemental
lock destro
druid resto
Mage arcane

My learnings for PvE:
I've tried healing with shammy and preferred the druid HoT method (+ boomkin didn't really work, shaman dps>druid
I've tried mage fire & frost and much prefer arcane
I've tried cat + enh shaman + retrib pally and it's great for quests (especially with pally in prot gear) but too many things to think about and dps wasn't great. If you plan on this you can play with the action target groups +IwT
I LOVE destro lock, almost as much fun as charlie hehe

All in all, a fun makeup, especially with portals/summons/drink.

I can't advise on PvP as a) I'm rubbish at it and b) I'm rubbish at it.

Can I ask why you're going with 4 and not 5? You could add in a tank and do instances?