Setting up external mixer 4 channels

Hi DJTT,

I recently bought a 4 channel mixer (DDM4000) and will buy 2 X1's too.
The meaning of this is:

I want to control 4 channels with traktor with an external midi mixer, and want to be able to prelisten to the 4 channels.
I can't find any decent screenshot to do this, and I don't want to buy a wrong soundcard.
I don't want to put too much money in my setup before I really know I want to DJ.

So I hope you guys can help me with the setup for this external mixer + traktor.

Do I need a audio 4 or audio 8? (Stereo is needed?)
And when I've got the proper soundcard, will I be able to prelisten, mix and use the effects of my DDM4000?

I'm sorry if these questions bother you cause they seem very obvious to a lot of guys probably, but I just want to be sure i'm not buying things that I can't use.

Thanks a lot guys.

WY.