As Mike points out, you may need a combination of speakers that are in parallel and series to keep the load around 8ohms. In parallel the resistance is the inverse of the sum of the inverse of each resistor. In series they just add. If you end up with a 2ohm load on your music source, you may burn up the output. Maybe they had something in there before with 8 outputs? As Merritt points out, the volume control will affect the overall circuit, so that needs to be taken into account as well. A volume control that is in series with a speaker, and they are both in parallel with another speaker/volume control, and that whole mess is in parallel with the same again, that should give you 4 speakers on one output. How well the music source deals with that depends on the quality of the music source.
Disclaimer: I don't deal with audio ever...just digging back to my electronics education from 1996, so I may be completely wrong here.
