4 inch union

I've been working on improving the ventilation for my laser in my craft room. I originally started with this [4" duct to 5.1" window vent adapter](https://www.printables.com/model/267556-4-duct-to-51-window-vent-adapter) from Printables. I liked the splines on the 4" side so I used that same pattern for the rest of my ducting. This model is a 4" union to connect two elbow together or to connect other pieces or tubing to the system. See my other designs for the elbow, fan mounts, and tubing extensions.