I'm not sure what you would do with the 3806 but on the 4806 you can specify how many speakers you have for the rear. Also, when you run Audyssey, it will pick up the number of speakers you have and their placement when it sends the signal out that particular terminal.
Wish I could be of more use but hopefully someone else will chime in. In either case, play with it to find out. Heck, that's fun in and of itself anyway. Congrats on your purchase and let us know what you think of your system once its up and running.
John
