youll never get 4 inches out of it. lowering a car is for handling not for looks. find a good set of springs and take what it gives you. if a company cant tell you how much it will lower your car then buy those springs.
if your doing it just for the looks of it, then just get ford racing c springs, and cut whatever else off them you want to get the look you want. also ditch the spring isolators.
Fox mustangs can't go down much more than 1.5" without big steering and driveability problems. 1 to 1.5" is about the best if you want good looks and reasonable handling.
You will need a bumpsteer kit, new shocks/struts and I recommend caster/camber plates.
I just installed H&R super sports. They dropped it about 2 inches all around... looks amazing imo. Drivability is still good... I can still clear speed bumps... dont care about handling... and i didnt to cc plates (dont need them didnt gain mauch camber). If you want 4 inches you will have to mod so many things... and would be easiest with bags... but you wouldnt be able to drive being that low.