diffrence between dual and single

A single voice coil driver has only 1 voice coil.  If it is a 4 ohm coil, that is what you have, a 4 ohm driver.  A duaal voice coil driver has 2 voice coils.  If it is a dual 4 ohm driver, you can wire it as a 2 ohm, 8 ohm, or a 4 ohm driver.  Wiring it as a 4 ohm will not handle the full rated power of the wooker. 

Dual voice coils are two independent wire coils wound on one bobbin. It was invented when bridgeable amplifiers were not as common as they are today (but you could still run both channels of a stereo amplifier into a single driver), and monoblocks were effectively unheard of. Today, however, they maintain their common appearance for the sole purpose of impedance flexibility. There are many other things that can be done with the secondary voice coil as well, but nobody ever uses them for anything other than a driven element. A single voice coil is just that... One voice coil wound around the bobbin. To use both channels of a stereo amplifier, you must have a second driver, or the amplifier must be bridgeable.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."