These are, as far as I can tell, the official Col. Jeff Cooper Four Rules:

1. All guns are always loaded (until you establish whether they are or not).
2. Never let the muzzle cover anything you are not willing to destroy. Keep your gun pointed in a safe direction at all times: on the range, at home, loading, or unloading.
3. Keep your finger off the trigger until your sights are on the target (and you are ready to shoot).
4. Be sure of your target. Know what it is, what is in line with it and what is behind it. Never shoot at anything that you haven't positively identified.

The Four Rules thread offers a critique of these and an alternate versions of rules 2-3 similar to the ones below.

The Four Rules that were here originally are not the standard Col. Jeff Cooper four rules, but here they are anyway:

1. All firearms are always loaded. ***
2. Always keep a firearm pointed in a safe direction.
3. Keep your finger off the trigger until you are ready to shoot.
4. Always be sure of your target and know what is behind it along the bullet's trajectory.

Miscellaneous other rules:

• If there are children around who haven't been taught to respect firearms, keep firearms in your personal possession or locked up.