We present experimental results on birefringence effects in an InGaAsP bulk semiconductor optical amplifier (SOA). Although the gain of the device is polarization insensitive, we observed a strong variation of the four-wave mixing (FWM) efficiency if the parallel input polarization of pump and signal wave was changed with respect to the device structure. This variation, which is attributed to birefringence in the SOA, can be as high as 10 dB for frequency detunings of about 6 THz. Thus, it might strongly affect the various applications of FWM for optical signal processing and parameter extraction. In addition, we performed polarization resolved measurements of the amplified spontaneous emission demonstrating different group velocity indices for TE and TM polarized light.