I have seen very few hitches with balls that are less than 4" from the bumper, but perhaps they exist.  I do know the two hitches I have with different size drops for different trailers are both more than 4" from the surface of the bumper.

We had two older trucks years ago with DMI bumpers with hitches that slid in and out, we slid them in when not pulling wagons.  Now, according to how I read the law, since they did not have balls, we could have left them out.