We had a customer that ordered the large size unit thinking it would fit a 4 inch vent. Its important to note that Critter Quitters on Home Depot and Amazon are measured in lead flashing size, not vent size. So a Size L fits a 4 inch flashing with a 3 inch vent, not a 4 inch vent. Those have 5 inch flashings and would require an XL. There are videos on Home Depot, Critterquitter.com, and the YouTube channel that shows how to measure a vent to make sure the correct unit is ordered.