This is an adapter for a 4in fan and hose. I wanted something for an exhaust system that I could put a fan in, so I made something. The outside has grooves to fit in an external house laundry type vent, and the inside part is a bit smaller to fit the hose on it. Both parts will need holes drilled where your fan bolt holes are, and it will need bolts. I will upload more photos once I get the fan in place.