That makes better sense. Understand that resent experience with cable modems that have phone and internet built in will probably introduce NAT issues for gaming if you connect another router to it. I'm not sure if you can bridge these modems to external routers like you can with DSL Modems.
I presume that your cable modem possibly has a built in router. I would check to see if the Cable modem has a DMZ. If it does, you could put the 4100 router on its DMZ so that the 4100 gets everything, then connect up the everything to it, including the xbox connected to the swtich.
If this works out, then review the gameing and gamefuel sticky in the 4500 forum for gamefuel setup on the 4100. The data information is the same, just the UI looks a bit different.
http://forums.dlink.com/index.php?board=144.0