Well, a 4 ohm svc sub is 4 ohms... simple enough. But, 4 ohms dvc, can be wired to 2 ohms, or 8 ohms. Dvc just gives you more options to work with you're amp. 2 ohms dvc can be wired to 1 ohm, or 4 ohms.
It's ohms law, if you have multiple loads in parallel, the impedance (resistance) drops accordingly, because there are more paths for current to flow. If you have loads in series, the resistance adds together to get a higher effective resistance. DVC subs allow you to wire for either, affecting the load presented to the amplifier and thus the power the driver gets.