The control of light group velocity via electromagnetically induced transparency (EIT) has opened up potential applications in photonic technologies such as information encoding, quantum processing, and optical switching. For traditional three-level atomic systems only allow light control within a narrow frequency range, limiting multi-channel applications. To overcome this limitation, this study focuses on multi-frequency group velocity control in a 87Rb atomic medium under the influence of an external magnetic field. By combining the semiclassical theory with analytical methods, we derive the expressions for the absorption, dispersion, and group index of a four-level atomic system. Based on these results, we investigate the influence of the magnetic field and laser parameters on the absorption, dispersion, and group index. The findings not only shed light on the mechanism for controlling multi-frequency light, but also suggest potential directions for developing flexible photonic devices that meet the future demands of multi-channel information processing.