After some research, I discovered that this was not entirely true. In fact, connecting a 4" dryer vent duct to a 4" exhaust pipe without a reducer may actually be more efficient and safer.
Firstly, using a reducer can create a build-up of lint and debris in the duct due to the reduced diameter of the pipe. This can increase the risk of fire and reduce the efficiency of the dryer.
On the other hand, connecting a 4" dryer vent duct to a 4" exhaust pipe without a reducer allows for better airflow and prevents lint build-up. Additionally, there are no restrictions in the airflow which improves the efficiency of the dryer.
It is important to note that the type of duct and exhaust pipe being used can also affect the efficiency and safety of the dryer. It is recommended to use a rigid or semi-rigid duct and a wall exhaust with a damper to prevent outside air from entering the duct.
In summary, it is not necessary to use a reducer when connecting a 4" dryer vent duct to a 4" exhaust pipe. In fact, it may actually be more efficient and safer to connect them directly. Ensure that the duct and exhaust pipe being used are of the appropriate type and include a damper to prevent outside air from entering.