The 4 hour rule for smoking meat is a food safety rule that means that the meat that you're smoking should pass through the "Danger Zone" (40°F to 140°F) from 40°F to 140°F within 4 hours.
Although the USDA also recommends getting the meat you're smoking to 140 F even faster to avoid growth of bacteria, and ideally within 2 hours for general food safety or cooling the smoked meat after cooking.
There's also another 4 hour rule for smoking meat, which is a technique for smoking brisket, which is also known as the 4-2-10 method, in which you smoke the brisket unwrapped for 4 hours, then smoke the brisket wrapped for 2 hours and then rest the brisket for 10 hours, which is a cooking method and not a safety rule.
The 4 hour rule for smoking meat when referring to safety ensures the meat stays safe from bacteria when smoking the meat.
Bacteria multiply rapidly between 40 F and 140 F or 135 F according to some guidelines by the FDA.
You have about 4 hours to get the meat from the refrigerator temperature which is around 40 F, up to a safe cooking temperature when smoking the meat, which is 140 F.
For cooked food, the USDA recommendation is that the cooked food or meat should be cooled from 140 F to 70 F within 2 hours, and then from 70 F to 40 F or below within the next 4 hours.
The reason why the 4 hour rule in smoking meats matters is that low and slow smoking keeps the meat in this danger zone for extended periods, so starting with cold meat and monitoring temperature is crucial.
When using the 4-2-10 brisket method when smoking the brisket.
Smoke the brisket at a low temperature in order to build a good bark and absorb smoke flavor.
Then wrap the brisket in butcher paper or foil to speed up cooking and retain moisture.
The allow a long rest period of 10 hours of rest and hold to let the brisket tenderize and the juices to redistribute, which is a longer rest than usual.