A second order four-wave parametric process proposed to be responsible for the generation of the broad up-shifted sideband in ionospheric stimulated radiation is studied by using a theoretical model and one dimensional electrostatic particle-in-cell simulations. The results indicate that (1) the initial evolution in nonlinear simulations is well described by the proposed theoretical model, (2) the simulation frequency power spectrum in a saturated nonlinear state exhibits important characteristics of the experimental observations and (3) the full nonlinear evolution of this four-wave process is crucial in interpreting the asymmetric nature of the wave frequency spectrum observed during experiments. ¿ 1998 American Geophysical Union