On the DynoIf you only look at the peak horsepower gain, shown in bold numbers, then sure, the headers only gained 4 hp at the wheels. But you'd be missing the large majority of what the headers really did for our '06 Mustang GT test car.
Study the dyno charts to discern the real benefits of the BBK headers: improvements in torque output. For example, look at the results at 3,000 rpm. The car in the baseline test made 265.6 lb-ft. With the headers, the figure jumped to 283.2, an improvement of 17.6 lb-ft.
Torque is what you feel on the street, and having more of it in the midrange is what's important for slipping through traffic and getting up freeway on-ramps. At various speeds, power output isn't too bad either. Again looking at 3,000 rpm, the baseline test result is 151.7 hp. With the headers in place, output jumped to 161.8 hp for a gain of more than 10 hp.
Additionally, recall that we're only testing the headers here. With BBK's crossover pipe in place, figure another 10 hp or so improvement across the board or well over 300 hp at the wheels on this car already equipped with a BBK throttle body and cold-air kit. Note too that the power and torque peaks with the headers were at slightly lower engine speeds, which makes the power all the more usable on the street. Furthermore, with the headers, the torque curve stayed over 300 lb-ft from 3,750 all the way to 5,000 rpm, another notable improvement over the baseline test.