There are many different types of quad rails. Do you want a one piece free-float? Those are the most difficult to install. UTG offers a 2 piece rail that doesn't require you to remove the front site. I installed one on a Beowulf upper and find that it retains zero just as well as a one piece free float. It's a bit heavy though.
You're gonna have to assess why you want the rail and then decide what will be the best solution.