How do I wire speakers/sub to get 4 ohms?

I'm new to car audio so I don't know much and will appreciate any help provided.  Right now I just have:

Dual XMP6000 head unit http://www.crutchfield.com/S-B4vZayuErfy/cgi-bin/ProdView.asp?g=300&I=070XMP6000

Two 6 1/2" eclipse 40w speakers in the back.  http://www.eclipse-web.com/integrated/se8365.html

Two stock 4" speakers in the front.

I want to power the two back speakers and a 12" sub with an amplifier.  So I was planning on getting a 4-channel amplifier and powering the two speakers and bridge the other two channels to the sub.

So my question is what type of 12" sub should I get and how can I wire it bridged to get 4 ohms?

Should i get svc, dvc, something other than 4 ohms?

Thanks for the help,

Kenny

So this produces 4 ohms?

Thanks for the help.