Analysis of modulational instability in optical fibers based on a phase‐matched four‐photon mixing approach is extended by taking into account higher‐order dispersion. Fourth‐order dispersion governs the modulation frequency in a region near zero‐dispersion wavelength, resulting in frequency saturation. The frequency decreases as group delay dispersion becomes dominant in comparison to the fourth‐order dispersion. To deepen the modulation, a phase‐matching condition for four‐photon mixing must be satisfied. A technique is studied which uses externally controllable birefringence of optical fibers to achieve phase‐matching at a given modulation frequency.