excellent read. The dual 4 ohm voice coil indicates the impedance of the sub. For instance you can have single 4 ohm subs, dual 2 ohms subs etc. This is used to give you wiring options so you can take full advantage of your amps power. For example many amps will give power ratings such as:

4ohm power = 250watts RMS
2ohm power = 500watts RMS

Just read the tutorial, it is a great explination.