I don't think it is mandatory, but in order to use VirtualBox it will need the following features (not sure if it has them or not):

* multiple shared networks between the machines (needed for clustering)

* parent/child VMs (differencing disks), otherwise you'll need a whole lot of storage since you need 5 VMs

The reason it's easy to use HyperV, is because you typically would set up a test environment like this using Windows Server, which has HyperV already installed. (I think if you have Windows 8 Pro you also have HyperV)