It is verified experimentally that the modulation frequency of modulational instability (MI) levels off to a certain upper limit by the fourth‐order dispersion of the optical fiber as the pump wavelength approaches the zero‐dispersion wavelength in the anomalous spectral region. Modulation frequencies observed in optical fibers with various dispersion values range up to 7 THz for an input intensity of 15 W/70 &mgr;m2. This agrees well with the recently reported theoretical prediction based upon the four‐photon mixing approach to MI.