this might get overwhelming quick--
computer audio can support 4,4.1,5.1,6.1,7.1, but all are surround effects--meaning that if the original source audio doesn't have any surround effects programmed, the other channels are not going to be used. in a 4 channel computer setup, the rear speakers will only work when the source tells the computer to send a signal to them.
car audio systems with 4 speakers send the same signal to the front and the rear speakers-- left front, and left rear will always be playing the exact same thing-- as will the right front and right rear.
to my knowledge, there is no onboard computer audio driver that automatically will take a 2 channel source, like music and make it play on 4 speakers.
but, there are programs that you can add to the computer to make audio play on 4 speakers, or you can go the easier route, and use a Y-Adapter to split the signal to the front and back speakers.