When applying the brake at highway speed my car will shift from 4th gear to 2nd.

It sounds like the car is going into safe "limp" mode. When you shut it off it resets the trans. While sitting still and the car is off, turn the key on-off-on-off-on and the trouble code will be displayed in the Odometer display. It will be a P---. Often times the solenoid pack in the trans or the shuttle valve in the trans valve body acts up and needs to be replaced or cleaned. Just depends on what the fault code is. Just take it to a good shop and use ATF+4 fluid ONLY, NO universal fluid etc...