The observation of spatial and frequency instability of an intense laser wave in sodium vapor at low-frequency laser detuning with respect to the D1 sodium transition is reported. It is shown that the observed picture of the scattering (cone radiation, generation of shifted sidebands, formation of filaments) in general is analogous to that at high-frequency detuning. A theory of resonance four-wave mixing in a three-level atomic medium is presented which is in qualitative agreement with the experimental results.