if you have one 4ohm dvc sub and you wire a positive from one coil to the negative of the othercoil,and the remaing positive and negative to the amp,the sub is in series which will give an 8 ohm load

if you wire positive of one coil to positive of the other coil and negative of one coil to negative of the other coil the sub is parallel and will see a 2 ohm load...if your amp is stable at 2 ohms then parallel the sub to recieve its max output