Hi Roy, is the pickup a humbucker, i.e. two coils. The 4 wires are probably 2 wires from one coil and 2 wires from the 2nd coil, the switch may have given the option of a humbucking pickup in one position and a single coil setup in the other, or another option would be a series humbucker in one position and a parallel humbucker in the 2nd position.
Do you have a multimeter? If so you could determine which coloured wires are connected to each coil. If you could post a photo of the pickup when not installed in your steel that would help to determine the wiring, also was the switch a 2 position switch?
Edit : just reread your post, it's a 3 position switch, so to me that would suggest a series humbucker, a parallel humbucker and a single coil option. If you could also post a picture of the connectors on the switch that would also help.
Edit2 : It could be a single coil with coil taps, i.e. the coil could be tapped at different points to enable different impedances to be selected by the switch, the higher the impedance the bassier the tone will be, a lower impedance will give a more treble tone.
A picture would be the thing
