It depends on the model of Charger the '04 Mustang GT is racing against.
A Charger with the 5.7 "Hemi" is going to be accelerate as fast as the Mustang GT if not slightly faster in a straight line - whoever gets a better launch is probably going to win a 1/4 mile drag race, they're that close. If it's an SRT-8 Charger, the Charger going to be faster than the '04 Mustang GT unless the Mustang has been modified significantly. The V6 model Chargers are not going to be as fast as the '04 Mustang GT in a drag race.