The output of an iPod is a headphone jack which is very low level. If you use a dock, you can get line-level but that is still very low (it's the level sent by any source device connected to a receiver or amplifier). So in essence you need an amplifier of some sort.
The easiest solution is a low priced receiver. With 4 speakers you'd be better off getting a surround receiver that can support multiple speakers. You could use a stereo receiver but it would likely require a speaker selector so that 4 speakers wired in parallel wouldn't present too low of an impedance for the receiver to adequately handle. At relatively low volumes, a simple stereo receiver with A and B speaker terminals could work with one pair wired to terminals A and one pair wired to terminals B with the switch set to A+B.