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I am having a problem where only one processor shows up in the windows 8.1 Guest. My system has an i7 Haswell processor and in the VirtualBox guest settings I have selected 4 out of the 8 under the processor options.

How can I get the guest to use multiple cores?

Windows 7's Task manager can show each core separately or one graph for all cores together. Have you checked whether 8.1's Task manager has such a setting?

One thing to remember is that your host has four physical cores, hyperthreaded, which shows as 8 logical cores as interpreted by Windows. Hyperthreading doesn't count when assigning CPUs to a guest. You should not assign all cores to a guest, or the host may choke one day. So the max you can safely assign to your guest on your host is three, and likely two or even one may suffice.

Check the Device Manager in Win8.1.