I was running with 1 prot warrior 3 shamans 1 holy paladin for the last few months of BC. Worked out really well tbh. Just had the pally pretty much spam FoL on the tank the whole time and since pallies never run out of mana with FoL spam I hardly ever even had to worry about healing on my tank. Simply had my shammies pop a 3 way Chain Heal when the group took AoE dmg and a shift modifier for my pally to cast HL should my tank actually drop low enough for me to use it. DPS was a bit slower than having a 4th dpser but again never really having to focus too intently on my warrior's health was quite nice. If I had to do it over again I probably wouldn't of bothered to level up the 4th shammy.