shakopcool forgot to mention that running an amp at 2 ohm loads will get hotter than if loaded with 4 ohms.
Here is the visual difference:
Imagine each speaker in the next two drawings as each coil (4 ohms) in the same sub. This is wiring in parallel. As long as the ohms are the same value, the value will be cut in half using this wiring method (from 4 to 2 ohm):
http://i81.photobucket.com/albums/j230/sparky3489/PARALLEL.jpg
Here is how to wire in series. The value will be added. Dual 4 ohms becomes 8 ohms:
http://i81.photobucket.com/albums/j230/sparky3489/SERIES.jpg
This is how you would wire two DVC (4 ohm) to get a 4 ohm impedance. This would be for bridging an amp with a 4 ohm load:
http://i81.photobucket.com/albums/j230/sparky3489/PARALLEL-SERIES.jpg
Here is the same wiring method using four SVC subs. Each sub is 4 ohms, the resulting impedance is 4 ohms::
http://i81.photobucket.com/albums/j230/sparky3489/SERIES-PARALLEL.jpg
If you want to calculate:
ohms in series. Z is the total impedance (ohms), and sub# is each sub(or coil).
Z = sub1 + sub2 + sub3 + ... for as many as you have.
ohms in parallel is a little tricky.
Z = 1 / (1/sub1 + 1/sub2 + 1/sub3 + ...) for as many as you have.
You can mix or match these equations to fit the needs of the amp by comparing them with the diagrams above.
Here is some info on boxes(and other tips):
http://spkrbox1.spaces.live.com
Go here for basic car audio research:
http://www.bcae1.com