In order to improve the handling performance of the vehicle, 4WS (4 wheel steer) started in Japan in the 1960s and has been developed to the active 4WS of today. From the 80s until now it has been applied to a relatively small number of production cars. The rear wheels of common 2WS car generate lateral acceleration from yaw angle which is caused by front steer, but the 4WS vehicle"s steer angle and lateral force of rear wheels can be adjusted. In addition, even if the driver"s steering angle is constant, it could adjust front steer angle of front wheels by varying front steering ratio. In recent, there are some 4WS cars encouraging yaw rate to follow the reference yaw rate by adjusting the rear wheel steering angle. 4WS system is integrated active steering system leading lots of advantages such as high-speed stability and rapid lateral acceleration response, low speed and reduced turning radius by using steering variables. There are many studies on handling characteristic advantages about integrated 4WS when it applies to vehicle actually. However, studies on boundary maximizing the handling characteristic when driver feels in practice are lacking. In this study, research about vehicle handling characteristics equipped with 4WS and when driver could feel stability, what range could be analyzed to maximize it, what types of vehicles are efficient to apply 4WS are proposed.