I have a http://www.newegg.com/Product/Product.aspx?Item=9SIA8UC39V0465. I'm trying to set the mouse 4 key (the thumb key that is in the rear of the 2 thumb keys) but I'm told that it is reserved. What does that mean and how/can I bypass it? I would really like to set it as my favorites key like I did in Skyrim.

It appears that mouse buttons 4+ are all "reserved", likely has something to do with game pad controller mapping. However, you can use your Logitech Gaming Software to program that button to be an F press while in FO4.