In terms of the dependence of non-linear susceptibility to polarization states of light fields,six kinds of combination existing among the above polarization states in stimulated four-photon mixing are put forward, analysis of the effect of polarization combination on the birefringence phase-matching is given according to the coupling theory and the frequency shifts for stimulated four-photon mixing in bended fibers are calculated. The explanation for "double-line" radiation under these modal combinations and the frequency fluctuation with the birefringence is given.