The recirculation door on the 4th gen Dodge Ram is a component of the HVAC system that helps to regulate airflow within the cabin. This door is located between the evaporator and heater core, and when open, allows hot air to be circulated back into the cabin. This can be beneficial in cold weather conditions, as it helps to warm up the cabin faster. Additionally, the recirculation door can also help to reduce outside noise levels when closed.