A system for a four-wheel drive vehicle for automatically changing a power transmission system of the engine of the vehicle from four-wheel driving system to two-wheel driving system in accordance with the steering angle during the turning of a corner. The transmission system is provided for transmitting a power from the engine to main driving two-wheel, and a fluid pressure controlled friction clutch is provided for transmitting the power of the engine to two wheels other than the main driving two-wheel. A steering angle sensor is provided for producing a steering angle signal dependent on the steering angle. A control system is provided to respond to the steering angle signal for decreasing the pressure of the pressure oil applied to the clutch thereby to cause the slip of the clutch during the turning of the corner.