with a single voice coil sub you cannot alter the ohm load the amp sees unless you wire another sub into the mix. To do that, or if you had a DVC sub you would change your wireing on the subs' terminals.
you have a 4 ohm dual voice coil sub...one sub with 2x 4ohm coils
one wire from the amp with a positive and negative
Parallel wireing = 2 ohm final load on the amp
take the positive from the amp and split it and run a wire to positive#1 and positive#2. Same is done with the negative.
Serries wireing, 8ohm final load at the amp
Take your positive wire from the amp and run it to the positive #1 on the sub. Run another wire from negative#1 to positive#2, then from negative #2 to the amp's negative. This will give a 'christmas light chain' type set up.
adding extra coils in the circut can work if you can find one that will handle the power. The problem here is you will be burning up the power to that coil strictly as heat. Basically, an amp will make 500w at 8 ohms and 1000w at 4 ohms. If you wire an 8ohm sub parallel to an 8 ohm coil the amp will make 1000w, but 500w will be wasted in the coil and 500 will go to the other sub. Basically it will do nothing for you and run your amp hotter.
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Like explained above, there is nothing you can do with the impedence if you have one sub. Thats why they make dual voice, so that even a setup with a single subwoofer will have a greater range of configuration.
Rockford Fosgate's website has a really useful calculator that will give you all the available wiring schematics for your configuration. Its a really good tool, give it a try.