A hydraulic-pressure control system for a four-wheel drive vehicle with a hydraulic-pressure operated transfer clutch, which is responsive to a controlled clutch pressure applied thereto to distribute a driving torque passing from a transmission between main drive wheels and auxiliary drive wheels, comprises a pilot-operated directional control valve connected to the transfer clutch for cutting off a supply of hydraulic fluid of the controlled clutch pressure to the transfer clutch and for directing hydraulic fluid of the controlled clutch pressure to the transfer clutch. The system includes a clutch pressure control valve provided upstream of the directional control valve, which pressure control valve is responsive to an external pilot pressure for producing the controlled clutch pressure, and a duty-ratio controlled solenoid valve for decreasingly adjusting the external pilot pressure to be output to the pressure-reduction valve in accordance with an increase in a duty ratio. A controller is provided for forcibly outputting a duty-cycle controlled exciting current of a predetermined high duty ratio such as 100% to the duty-ratio controlled solenoid valve to reduce the controlled clutch pressure to a low pressure level (preferably a minimum clutch pressure) when the directional control valve is maintained at the cut-off position in case of 4L range wherein the transmission is loaded heavier.