I am not 100% familiar with the Sportster 4 unit, but are you sure you are not talking about the AUX OUT on the car docking station?
Too my knowlegde, and I could be wrong, those units have an AUX OUT on the car dock. They are not AUX IN.
The AUX OUT on the car dock would be used to direct connect the unit too your radio headunit in your car either by:
1. Direct connect FM wired modulator(this is the worst direct connect type IMHO)
2. Car has a cassette deck built into the radio, and direct connect a AUX cassette to the unit for playback of satellite radio content(Very good results. Clear reception. Just that below connections tend to sound a LITTLE BIT BETTER IMHO).
3. Your unit has an AUX IN on it's front panel so you can connect the AUX OUT from the Sirius car dock to the radio using a 3.5mm connection(on both ends. Usually 4-6ft in length ample. Found at Best Buy/Circuit City/Sears/Radio Shack/etc... pretty cheaply) EXCELLENT WAY to connect car dock to radio for clear reception, and excellent sound.
4. Using a 3.5mm connection on car dock end, and Red and White RCA plugs for REAR AUX INPUT on Car radio. Again, excellent way to connect radio.
I have never heard of these ever having an AUX INPUT on their car docks, only AUX OUTPUTS for direct connection to the car radio to avoid wireless FM transmission.