Most of the aftermarket 4 piston calipers have less piston area than the stock Mopar single piston calipers. So to work correctly, you need a master cylinder with a smaller bore size. (you also need to reduce the wheel cylinder size out back to get things back into the correct ratio but that is another can of worms)

Bottom line is that you can't just randomly change parts of the brake system and expect the stuff to work correctly.