Why are refrigerators 4 degrees and freezers -18 degrees?

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 iceboxes 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 degrees must be an engineering compromise, a good temperature for long time storage but not too expensive in power consumption to retain.

In any case both refrigerator 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.