Quote Originally Posted by Djaria
I went the way of editing the config.wtf of each client.

First client:
SET processAffinityMask "1"
Second:
SET processAffinityMask "2"
Third:
SET processAffinityMask "4"
And fourth client:
SET processAffinityMask "8"

This way each client uses one CPU and I don't need any other applications. Works like a charm for me. Load is distributed evenly over all cores. Here is a thread that gave the hint:
http://forums.worldofwarcraft.com/th...cId=1778017311
This will work well, however if you set it so all the instances of the game can use all 4 cpus, then you will get better performance, especially if you have graphics and other options turned down on some of the instances of the game.

Windows will do a fantastic job of balancing the load between the CPU's, but by letting each instance of the game only have access to 1 cpu each, you are severely gimping each of those instances. You may have some instances of wow only running at 20% cpu usage while some are pegged at 100% and giving you reduced framerates. By allowing all of them to have access to all 4 cores you can be sure you are using your 4 core CPU to the max.