In modern United States coinage, there are no incuse design features. All are relief. When any incuse feature is seen, the likelihood of that feature having occurred in the manufacturing process is extremely low. The mark you see as a "4" is simply damage from being struck by something. A hit. Marks like this are common on cents due to softness of the alloy in comparison to the alloys of nickels and clads. The modern dollar coin is softer than the nickel and harder than the cent, making it nearly as susceptible to damage as the cent.
You still need to learn about the minting process. It will make spotting errors vs damage much easier.