i've been looking to get one of those rectangular dryer vents to my FL dryer. the normal connector calls for a 4 inch round but i would love to shorten it by 2 inches as i'm working with a very small room.
You can buy 4" metal duct and adjust it to the contour that you are looking for. The walls of the duct are thin, and will yield somewhat. You can also go to a sheetmetal shop and have them build the exact piece that you need.