The analysis and design of a computer-controlled system to coordinate the steering of the four wheels of an advanced vehicle system is presented. The application of modern control and estimation theory produces a Linear Quadratic Gaussian control design incorporating a directional dynamics model of the vehicle and models of the front and rear steering dynamics. Minimization of a performance based cost functional reduces sideslip angle variations, lateral acceleration and yaw velocity response times, yaw velocity and roll angle oscillations, and the use of dynamic steering control. The measurement system estimates the states for full-state-feedback from a set of practicable and realizable measurements, enables the development of the vehicle implementation and the eventual reduction of sensors needed, and adapts to changes in vehicle trim elevation and loading condition. Gain changes as a function of forward velocity attempt to maintain sideslip angle minimization, response time, and control sensitivity independent of vehicle speed. Simulation results illustrate the advantages of active four-wheel-steering over two and proportional four-wheel-steering.