A question on cooling/refrigeration (details inside)

I just got a new refrigerator (new to me anyway) and it has a digital readout on front. The preset temperatures from the factory are -4 degrees F for the freezer and 38 degrees F for the refrigerator portion.

My question is why? Why is the freezer -4 degrees? Why not -10 or 0 or anything below 32 degrees? It would still be frozen would it not? Is it based on some scientific principal or method?
Does it have to do with safety or efficiency or energy savings?