A four-wave mixing experiment in a bulk InGaAsP traveling-wave semiconductor amplifier is reported. The maximum pump-probe detuning is 4.3 THz. The equivalent time resolution of 37 fs is high enough to measure with good accuracy the time constant of spectral-hole burning (100 fs in our case). The simultaneous presence of spectral-hole burning and of an instantaneous, within our time resolution, saturation process is clearly displayed.