Interaction of four light waves in a nematic liquid crystal is studied theoretically. We graphically on the phase plane solve equations describing four wave mixing (FWM) under excitation of the static lattices of the dielectric permeability in the nematic liquid crystal. We show existence of unstable eigenmode in FWM. Effect of optical switching of energy is predicted. Energy exchange among the light waves depends on relative initial phase among the four waves. Experimental conditions, under which the effects that are studied would be observable, are discussed.