Non-perturbative phenomena in four-wave mixing spectra of semiconductors are studied using the exact solution of a widely used phenomenological non-linear equation of motion of the exciton polarization. It is shown that Coulomb interaction, included in the nonlinearity, leads to two characteristic effects, which are essentially of dynamical origin, -- a split of the exciton peak and a non-monotonous dependence of the response at the exciton frequency on the magnitude of the external field. Relations between the spectral features and the parameters of the system is obtained. It is found that the transition from perturbative to non-perturbative regimes is controlled by parameters inversely proportional to the decay rate. It implies that the condition of low excitation density does not necessarily warrant applicability of the perturbational approach.
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