Four-wave mixing was performed in a multi-quantum well semiconductor optical amplifier in order to demultiplex 2 ps pulses from 80 GHz to 10 GHz. The demultiplexed pulses were measured using frequency resolved optical gating. This technique allows for full characterization of the pulses in both the time and frequency domains. The measurements confirm that the four-wave mixing conjugate signal preserves the phase of the injected signal. Furthermore, in the time domain the influence of the pulse pedestals present on the injected probe pulses is significantly increased on the output four-wave mixing signals. This effect is attributed to the nonlinear suppression of the device gain.