Not software I use, I'm afraid, but you might find this online help page
relevant. Note that in the CPUs To Leave For Other Applications
section they say
Set this value to a number other than 0 to prevent After Effects from using all of the CPUs (processor cores) in your computer system. For example, if you have a computer with 8 CPUs, setting this value to 2 leaves 6 CPUs for After Effects.
It may be that After Effects CS4 is ignoring those virtual cores because Windows identifies them as such and the imacs OS doesn't but I'm guessing wildly.
In any event earlier on the same page it states that
Optimum performance is achieved with computer systems with at least 2 GB of installed RAM per processor core.
So you actually might achieve better performance with your 6GB of RAM if you set the Memory & Multiprocessing preferences
to limit the number of cores to three rather than the four you have or the eight you want. You should be able to see whether three or four cores is quicker by timing the same job.