On the practical side, if you live in an apartment, or anywhere else without a safe backstop, you need to build a "loading and unloading" station.

A cardboard box full of a couple feet of old books would suffice for most handguns (pointing down into it, not level at the wall of course), or a 5 gallon bucket full of sand (not rocks obviously), or even a bullet-resistant panel, there are commercial ones sold for just that purpose.

In a detached home, where you know the location of the occupants, it's your carpet, have at it. In most ranches the crawlspace dirt will stop anything that penetrates the floor anyway.

The Four Rules absolutely prevent injury as long as any single one is followed, even if three others are broken. The only one that demands something external to the gun owner is the safe backstop and it is their responsibility to ensure one is available.