Personally, when I smoke a butt, I'm making pulled pork. I see no need to inject given that I'll be shredding the pork into little bits, and mixing some sort of sauce into it. Given the amount of fat that is spread through a butt, it will render nicely and keep the meat moist during the smoke. I smoke my butts for 12-20 hrs, and if I was to inject, it would completely screw up the 4 hr rule.