A degenerate four-wave mixing experiment is performed in the saturable absorber of cryptocyanine. When the pump-beam intensity is below the saturation intensity, the reflectivity of the probe beam does not change, even though the probe-beam intensity is increased; however, the reflectivity begins to decrease slowly for moderate pump-beam intensity and finally decreases as l/q (q is the ratio of probe-to-forward pump-beam intensity) for pump-beam intensity far above the saturation intensity. The results are explained by extending Kogelnik's coupled-wave theory of volume grating to the four-wave case.