  We investigate the modulation instability of multiple four-wave mixing
arising from a dual-frequency pump in a single-mode fiber or waveguide. By
applying the Floquet theory on account of the periodic nature of four-wave
mixing, we reveal a collective type of instability occurring in the anomalous
dispersion regime. Our interpretation of the linear stability analysis is
validated by the numerical solution of the nonlinear Schroedinger equation
