We have already proposed a feedforward type control system for the four wheel steering (4WS) vehicle using the model following control theory. The control system is designed on the assumption that the input is the steering wheel angle. In this paper we deal with the stability of the 4WS vehicle. Generally, it is considered that the driver steers a vehicle by both steering wheel angle and steering wheel torque. We carry out an investigation of the pole location and a computer simulation for the 4WS vehicle as a steering wheel torque input system that has a feedback path to the control system through the dynamics of a steering system. Furthermore, proving ground tests are conducted with a real 4WS vehicle. The results show that the feedforward control system has an effectiveness to stabilize the vehicle motion.