Hmm, I haven't had much experience with extenders however I presume it should gain an IP address from the 4500? If it does then I would also set that up as a static IP as well. You might have to call up and get support help on the extender and make sure it's being setup well. Ya, I would use a wireless mode only if at all possible. WiFi is steadier when using one mode so it only has to handle that mode instead of 2 or 3. Specially in gaming. My buddy in OK has his on on G mode only at 2.4Ghz and we play well all the time, even while dad is online. I presume once the 4500 is set up well, all that is needed will be to get the extender set up and working well.
Let us know how this goes.