Normally 4wd cars run higher kv motors because they need more power. while the torque is higher on a lower kv motor the hp is lower. there is a mathmatical equasion to figure horsepower that I dont remember but the jist of it is the torque of a motor is multiplied by rpm then divided by another number to give you the hp rating of a motor.

Higher kv motors make up for thier lack of torque in the gearing because they spin so much faster than the lower kv motors. Even though they have less torque they feel like they have more torque when geared properly and will be faster as well because of the increased rpm range of the motor.

Most 4wd off-road classes in racing are mod classes while most 2wd classes are stock (though there are exceptions to these rules) This is just talking about 1/10th scale of course. Moving up to larger scales (like 1/8th) the KV ratings go down but this is because the motors are much larger and produce more power (in part because of the increased voltage normally used) so there are exceptions to those rules as well.