If you get a 6" fan instead of a 4" fan, then you will likely need two Doran louvres to provide ample airflow. If you calculate the measurements, one louvre seems like it would be enough (64 square inches is more than twice the area of a 6" circular exhaust). But because of the design of darkroom louvres they do impede airflow somewhat with internal baffles, probably by around 50 percent. For this reason I had to use two louvres to get enough airflow.

And no prob, glad to help!