Hyper-V and Windows OS license

The four vitual guest licenses only applies to Enterprise editions of server 200x. Standard edition with HV give you one additional license regardless of your VL/EA status. It applies to Retail as well. Datacentre editions of Windows of course let you run an unlimited number of licenses (hardware pending).

So you can run your Windows host with Hyper-V, and a single guest machine all from the one "license", so there is no "extra" copy you need to buy at all and you can keep your core or host install clean of any other apps or services, then load up as many *nix guests as you want at no additional cost.

Of course, it's all still a complicated mess, but it is getting better for the Windows world and dragging VMWare along with it.