Frequency conversion is considered as a key functionality for wavelength division multiplexing systems, which converts an incoming optical carrier of one frequency to an outgoing optical carrier of another frequency while preserving the quality of carried data. The all-optical approach is promising for such a purpose because not only the system complexity and power consumption of frequency converters are greatly reduced but also the flexibility and reconfigurability are highly improved. Four-wave mixing of solitary semiconductor lasers has been proposed to achieve all-optical frequency conversion. Although the conversion efficiency is relatively high, however, the conversion bandwidth is small, limiting the conversion range, and the efficiency flatness is poor, distorting the data signal. In this study, we propose to use four-wave mixing of injection-locked semiconductor lasers instead. When a semiconductor laser is subject to a strong optical injection, it can enter into stable locking dynamics before undergoing Hopf bifurcation. An incoming optical carrier of one frequency perturbs the injection-locked laser, generating an outgoing optical carrier of another frequency through four-wave mixing. Tens to hundreds of gigahertz of frequency conversion can be achieved, increasing the conversion bandwidth by 3 folds. The conversion efficiency varies within 5 dB over the enhanced bandwidth, improving the efficiency flatness by at least 10 dB. Greatly improved eye-diagrams and bit-error ratios are thus obtained. The input power dynamic range of the incoming optical carrier is greatly enhanced, increasing the flexibility of the proposed system.