I think it's ok to shift back and forth between D3 and D4. Tho it's written in the manual that you can even shift between gear 1-4. In fact my friend who works as a technician at a BMW dealer says that it's perfectly fine to shift, just dont downshift that much when you are at high speed (ex. >70mph)

Starting the car from 0mph at D3 is already much more powerful than at D4. I wouldnt use 2 or 1.