Are these power brakes? 4" is a lot of play, I think you need to first visually take a good look and see where the play seems to be coming from. If you have a booster does it seem like you are getting good assist out of it? There could just be a lot of air in the system yet, did you bench bleed the MC? If you have power brakes press the brake pedal with the engine off and then while holding the pedal down start the engine, you should feel the pedal move down once the engine starts if the booster is working properly.
Clyde.........75 Monza 2+2