In the places where I see four-way stop, there is some constant but not heavy traffic during parts of the day, so if you had a primary / secondary road with only a two-way stop, the secondary road would not flow well. However, the traffic was not high enough to warrant a signal.

This makes sense to me, and I don't see how other systems would be anything but worse. A signal would have a short cycle and only delay people during large parts of the day, a two-way stop would cause problems in one direction, it seems excessive to tear out a bunch of road to put concrete in, etc.

It is of course very natural that the roads in the US are built to completely different standards than Germany, given how incredibly different the layout of US cities are from German ones, broadly speaking.