A four-wheel drive work vehicle includes an engine, a pair of right and left steerable wheels rotatably driven by receiving power from the engine via a differential mechanism, a pair of right and left non-steerable wheels rotatably driven by receiving power from the engine via right and left side clutches and a pivotal cam mechanism for operating a clutch operating member for controlling the side clutch based on a steered displacement of the steerable wheels. The pivotal cam mechanism includes a cam member which is displaced based on the steered displacement and a cam follower member operably associated with the cam member. The clutch operating member is displaced via the pivotal cam mechanism by a predetermined amount in response to a steering operation of the steerable wheels by an angle exceeding a predetermined angle from a straight traveling condition and in association with the displacement of the clutch operating member by the predetermined amount, one side clutch for one of the non-steerable wheels located on the inner side of the vehicle turn is automatically disengaged against an engaging urging force.