In complying with a customer demand for improving low-speed maneuverability of commercial vehicles in narrow streets, a medium-duty truck with a mechanical linkage type four-wheel steering system with a hydraulic assist and a steering lock device is developed. A mode select gearbox allows a driver to select one of three rear-wheel steering modes; 2WS, same-phase 4WS, and opposite-phase 4WS. The steering lock device is locked during 2WS operation for preventing rear-wheel steering. An electronic control system is applied for easier mode selection, synchronization of locking and unlocking the steering lock device with a mode select operation, and vehicle speed limitation during 4WS operation. We made efforts particularly to suppress vehicle yaw motion when the vehicle is running in the same-phase 4WS mode. Several innovative new mechanisms are incorporated on this vehicle. This paper deals with these mechanisms and these functions.