Basically, 4WD does not introduce power oversteer. However, most people still prefer it simply because it provides superior cornering grip thus improve cornering speed. As I have promised earlier in the Cornering Grip section, here I'll explain how 4WD improve cornering grip :

Consider a driving wheel running in a corner. Due to the frictional force applied from the road surface, the tread in the contact patch distorts and creates slip angle. The faster the car corner, the more centrifugal force generates thus the larger the slip angle becomes. You can interpret this as the elastic distortion of the tire generates a counter force to keep the car fighting with the centrifugal force. When the car is accelerated fast to the extent that the elasticity of the tire reaches its limit, it could not distort anymore, thus more speed will lead to the tire slide, and the car lose grip. This point is what we call "Cornering Limit".

A FWD or RWD car has already a lot of tire distortion (slip angle) in the driving wheel because the tractive force is shared by only two wheels. Therefore there is not too much space left before the tires running into their cornering limits. On the contrary, 4WD cars distribute tractive force to all wheels, thus each wheel shares considerably less tractive force thus create smaller slip angle in cornering. The car can corner at higher speed before the slip angle reach the cornering limit.

There is always argument that whether the neutral steer of 4WD is better than RWD's oversteer. Although neutral is more favorable in the entry phase and mid corner phase during cornering, it doesn't provide the "correctability" of power oversteer in the exit phase. Remember, no driver could avoid miscalculation, no matter Mrs. Robinson or Michael Schumacher. Normally we need to feel the car's attitude and the road condition every moment before deciding how to control the car in the next moment. In this sense, RWD's controllable power oversteer is what we want.

Moreover, power oversteer of RWD ask the driver to intervene the throttle during cornering. This let him feel more involving and that he is mastering the car. In contrast, 4WD cars let the tremendous grip, the limited-slip differential and even the computer to rule the car's cornering. Therefore we always hear road testers said RWD is more fun to drive.