Go into your host's BIOS/EFI setup and check whether Hyper-threading (HT) is enabled. If Workstation is complaining about insufficient cores when "Total processor cores" is set to 4 in your VM's configuration, it's likely that your host has Hyper-threading disabled, so only two cores will be visible to the host. Enabling Hyper-threading will provide four cores. (Enabling HT will probably get VBox working too...)
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Darius