I would go for:

1 Tank, 3 Mages, 1 Priest.

If you could level a 4th mage on the tank's account you can bring him in when your going to play a battleground or arena, if you want to do heroics you bring the paladin in.

I suppose you're going to RAF so you could easily boost a 4th mage to 60

Kind regards,

Steven A.k.A. Totemuc