I have "Sound Blaster X-Fi Surround 5.1 pro" sound card but only four speakers attached to it. (front left/right and back left/right)
But the game, by default, use 5 channels (those four plus center speaker).

In sound card driver settings i can choose 2.1 or 5.1 only.

How to force the game to use 4 speakers only ?

If you have the speaker icon by your clock, right click it and select playback.

Then in the open window, select your Speakers, then at the bottom click Configure. Here you can change your windows speaker setup. In the options, you can choose 2.1 Quadraphonic or 5.1 (usually).

You want quadraphonic. You can't make skyrim use a custom speaker option, but you can change the windows speaker settings each time you play.

I tried this, but i have not "Quadraphonic" option, only 2.1 or 5.1.
That is why i ask for help.

I have a Soundblaster also, and don't have this option. It was there when I was using my onboard Realtek audio though lol.

In your Start menu, go to Creative, and you should be able to go on to Creative Audio Control Panel (I can).

From there I'm able to select 4 speaker setup. Try it.

I tried "Creative Audio Control Panel" with no luck. There are only two options too (2.1 or 5.1).

Maybe some registry tweaks can be done ? (for the Game or for the Sound Card driver)

I'm stumped to be honest lol.

Have you tried updating your sound card driver? Have a look on the creative website for the latest one.

I checked it out. I have latest driver from those site.
It seems I caught unique issue. Nobody play with such external usb sound card.