well the agro part wont be an issue wether you take 1 or 4 imo., if anything having more alleviates the stress a bit as each SP generates the same amount of agro, but if you have 4 times the potential healing for the same threat then great. the only time this would be a problem is if you are going all out on a boss and all your SPs end up miles above the Tanks threat lvl and thus the boss goes and picks one off at a time. If the boss is only a weak hitting one, then this shouldn't be a problem come epic gear

don't forget if you have a pally in the group you have 30% less threat though his blessing and then an additional talented threat reduction.