The D3 is used when climbing up a steep hill it will shift your car to a lower gear so your rpm's climb and you can maintain constant speed also use this when going down a steep hill as it will slow down your car, reduce wear and prevent overheating on your brakes. D3 is also useful in stop-and-go traffic because it will keep your transmission from switching back and forth between 3rd and 4th which burns more gasoline.
and D4 well that opens up all the gears for all your other situations. I didn't copy and paste :roll: