You should know that when you twist + and - speaker leads from two subwoofers together, the ohm impedance will be cut in half. Thats one way to wire for a specific ohm.
So if you did that with two single voice coil 4 ohm subs, the amplifier would see a 2 ohm load.
You can also raise ohms wiring + to - on the other sub, and then take the other terminals and wiring those left over to the amp, you would raise them to 8 ohms.
There are alot of variables when talking about wiring specific ohms.
for instance dual voice coils and single voice coil subs.
Heres a cool guide to use if youre ever in doubt....
http://www.rockfordfosgate.com/rftech/woofer_wizard.asp