As the title says this is a topic to get the achievement "The Dedicated Few" Easier, on the achievement it says you need to kill all bosses of naxxramas with less than 9 or less than 21. None of these achievements include The Four Horsemen needed to be killed.
So if you are trying to do this achievement and your having a hard time killing The Four Horsemen with less than 9 or 21 you can bring some extra in for just that boss and still get the achievement.