With 4 boxing I find questing and then running instances my level once or twice works pretty well. Grinding due to being grouped seems the slowest way to go, thou killing is pretty quick. With all the changes Blizzard made to quest and leveling experience not questing in my opinion is hurting your leveling speed. I have 3 70’s (Paladin, Druid, and Priest) as well but have not tried boosting. My goal is not to race to 70 but learn to control my team as I 4 man instances.