I assume that the refrigerator's temperature of 4 degrees has something to do with the fact that water is densest at that temperature. Does that inhibit microbe growth? But what about the freezer, what is special about -18 degrees? Is it a trade off of some type?
The 4 degrees is the logical minimum for a refrigerator because below that ice crystals start forming which many time destroy the texture and taste of many foods. Of course the lower the temperature the more inactive the decay bacteria and molds, but who wants a wilted salad.
The first refrigerators were filled by ice bought by the iceman every day so 4 degrees was the minimum that could be obtained.
In modern refrigerators a balance is struck, having various drawers for fruits and vegetables where the temperature is higher so they retain their texture and appearance.
Freezers are used for long time storage of food that is not destroyed by freezing, either in appearance or taste or texture in order to inactivate decay processes. In countries where freezing temperatures prevail in the winter it was observed that meat and fish etc retain their food properties well when frozen and can be kept for long months. The -18 must be an engineering compromise, a good temperature for long time storage but not too expensive in power consumption to retain. One also must keep in mind that ice grows as temperatures plummet so a compromise also must be found with the destruction of nutrients by the enlargement of ice crystals if temperatures are arctic.
In any case both regrigerator and freezer have a thermostat that could be set for higher temperatures.
I believe that most refrigerators are 'tunable', so in most cases you can get to negative temperature inside refrigerator, but this has significant negative effect - lots of ice grows on the walls.
So, smallest positive temperature is the best for refrigerator.
4C does not 'inhibit' microbe growth, it rather slows down all chemical reactions, AND slows down any microbe/bacteria work (including growth).
As for -18C - it's also usually tunable & depends on the work medium used in freezers. Different kinds of work medium (CFC/Freon's, Ammonia) allows to reach different temperatures.