An axle flip alone will drop the rear the thickness of the axle plus the thickness of the spring pack. Since you are now gonna have the axle on top of the springs.
A longer shackle alone will drop the rear only HALF the difference of the stock shackle versus the longer shackle. A 4" longer shackle by itself will only drop the rear 2" since you are only dropping one end of the spring pack.
If you have a 4" longer shackle AND a 4" dropped front spring hanger then you would get 4" drop in the rear.