What is the primary function of a 4" coupling cap?

The primary function of a 4" coupling cap is indeed to cover and seal the end of a hose. This is important as it prevents contaminants from entering the hose, protecting the integrity and functionality of the firefighting equipment. By sealing the end, it also helps maintain pressure within the hose, ensuring that the system operates effectively when water is needed.

Using a coupling cap in the field is a standard practice for keeping hoses safe when not in use or when they are part of complex configurations where the end of the hose may be exposed. This function is essential to ensure that hoses are ready for use at a moment's notice without the risk of damage or debris getting inside. In contrast, the other choices involve activities that a coupling cap is not designed for, such as connecting hoses, increasing diameter, or directly extinguishing fires.