An optionally connectable four-wheel drive vehicle provided with an engine having a drive shaft, two main drive wheels, a main power train permanently connecting the drive shaft to the main drive wheels, and in turn comprising a transmission and a main differential, two normally driven secondary drive wheels, and an optionally connectable secondary power train for also connecting the drive shaft to the secondary drive wheels, and in turn presenting a gear drive, at least one secondary clutch, which is connected on one side to the drive shaft upstream from the transmission, and on the other side to the secondary drive wheels, and a unit for the reversal of the motion, which is controlled to reverse or not to reverse the direction of the motion.