Sparks, there's two problems:
1. Driver aids. In the "settings" menu when you are at a race track, you can go in there and turn off driver aids, which will help the RWD cars get the tail out. Unfortunately you can't do this in the license tests, only when you are in one of the race events.
2. Tires. The cars are issued with what GT4 calls "Sport Tires". These are some kind of super sticky rubber that gives road cars 1.5g cornering ability and very little wheelspin. When you buy a car, go into the tuning shop and get the N1, N2, and N3 tires. These will give grip and traction levels much closer to the real world, and you will be able to hang the tail out on the RWD cars.