Hi Sam,
I think that you can easily do this by shutting the machine down and editing the settings in VMware Player. In the VMware Player window, when the machine is shut down, you can "Edit the Virtual Machine Settings." When you get into the settings, you can select the number of CPUs that the virtual machine should see. I think it should be not more than half of the number of physical cores in the system. So, you have 4 cores, then you should set this to no more than 2. Feel free to google around. There are a number of articles online about this. It doesn't hurt to experiment either.
Hope this helps!
John
