Boosting your characters through instances with a lvl 70 char will possibly be the fastest way of leveling...
Though i cant really tell you how good a 70 shaman is in boosting. Its kinda lacks the Aoe part a mage or a prot paladin has, But if you spec it enchancement and take a couple mobs at a time ( instead of the entire instance ) i cant see why it wouldnt work

If you also consider using your multiboxing group for pve aswell, i would go for the 4 shaman + Tank setup. The experience you can get in the lower instances about how to control your characters will pay off when you ding 70 in both PvE and PvP.