We present a theory of frequency degenerate four-wave mixing in semiconductor microcavities in the strong coupling regime. The intensity and polarization state of the signal wave are obtained from the probability of the two-polariton scattering on the anharmonic potential arising from the fermionic character of the exciton constituencies. We show that polarization sensitive four-wave mixing measurements enable one to discriminate the contributions to the excitonic anharmonicity due to the repulsive and attractive interactions between excitons and the phase space filling effect.