Don’t believe I’m aware of the “4 gun rule”.
From
Jeff Cooper webpage on Wikipedia Firearms safetyCooper advocated four basic rules of gun safety:
All guns are always loaded. Even if they are not, treat them as if they are.
Never let the muzzle cover anything you are not willing to destroy. (For those who insist that this particular gun is unloaded, see Rule 1.)
Keep your finger off the trigger till your sights are on the target. This is the Golden Rule. Its violation is directly responsible for about 60 percent of inadvertent discharges.
Identify your target, and what is behind it. Never shoot at anything that you have not positively identified.
Also covered in the above writeup are the
Color Codes (White, Yellow, Orange, and Red) and firearm
Conditions, e.g., Condition 1: mag inserted, round in chamber, safety on.