When you say faster, do you mean faster as in a higher top speed? Or faster as in quicker lap times? Because an S4 (with the right setup) turning in quicker lap times than a Mustang is understandable.
actually i believe power/weight ratio is better in the stang....
when i say faster i mean altogether, its 0-60 is about 2 seconds better (its about 3.2 for the S4), quarter mile is better, 1000m is better, top speed is better as well.