As Mark suggests, the fourth pin is the centre tap, so wire the end terminals (pins 1 and 3) to 5 V and ground, the wiper (pin 2) and centre tap (pin 4) to two Analog inputs. You then successively read the wiper and centre tap voltages, and if they differ significantly, act accordingly.
jackrae:
It may be a 'sine' pot with three inter-connected windings. The centre connection point may be the common or wiper connection. The wiper then tracks around a circular resistance which has three fixed connection points around its periphery. The resistance measured between the common point and the other three connections then defines where the wiper is in respect to the X-Y plane.
No, totally different function - what you describe is a continuous rotation "synchro" pot used for remote indication.
It had five pins and two wipers 180 degrees apart.