To assign more than 4 processors, you need to do the following:
1) In Hyper-V Manager, shutdown the VM
2) In windows explorer, browse to the VM then open a subfolder called Virtual Machines.
3) Edit the .xml file that is in the folder. Locate the following xml code and change the number as required:
4) In Hyper-V Manager, under the settings for the VM, the GUI should now display the number of processors specified in the .xml file.
5) Start the VM.