It is very possible to run 4 instances of wow on a single computer. I am new to this myself having gone to 3 boxes a few days ago and just jumping up to 5 tonight - however I have succesfully completed the 1-10 area with my little group. I would be a bit concerned about trying to run even 4 instances on only a dual core machine. Definetly look into a Quad Core. 5 Instances put my machine at 85% CPU utilization constantly with some higher spikes. My system specs are below if it helps.