I have cooked 4 or more butts at once on several occasions. If you have success with one, you'll have success with four. Just figure 2 hours per pound (the largest butt, not total!) cooking time as usual and leave extra time for prep, both before and after cooking.
I would add, if you are using the Minion method for starting, you better use nearly a full chimney. That much meat needs lots of energy to get warmed up, so you should use more lit charcoal.