After hours (erm, minutes) of struggling with the start /affinity cmd thingy, I couldn't get it to work with wow.exe. Always ended up with two CPUs. It did work with notepad, however, so I figured it might have something to do with wow config files. I took a look at WTF/Config.WTF and found this:

SET processAffinityMask "3"

Figuring it's a hex value, I changed it to "F" and whaddya know, all 4 CPUs active on start :p (Edit: After some further testing, I noticed the value is actually decimal, so replace the F with 15. If I sticked any text in there, it used all four CPUs so I got tricked since I didn't do any testing besides "F" :p)

If you can't find the command in your config file, I don't think adding it would hurt :p
(Edit2: blergh, read this before you do http://forums.worldofwarcraft.com/th...78017311&sid=1 (so you'll have someone else to blame when your pc explodes!))