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