Final answer:
The 4th firearms safety rule is to 'Be sure of your target and what is beyond it', requiring shooters to be aware of their surroundings to prevent unintended harm.
Step-by-step explanation:
The 4th firearms safety rule is "Be sure of your target and what is beyond it", which emphasizes the importance of being aware of not only the target but also the environment surrounding it to prevent accidents. It is vital to recognize what might be behind your target before you shoot, as bullets can travel significant distances and penetrate various materials, potentially causing unintended harm.
This rule is part of a larger set of guidelines meant to ensure safe firearm handling and use. Other rules include treating every gun as if it's loaded, keeping your finger off the trigger until ready to shoot, and never pointing a firearm at anything you're not willing to destroy.