You first need to establish which type of sensor it is, having 4 wires does NOT mean it's a zirconia, it could just as easily be a titania.
If it has a RED wire it IS a titania, if it has no red wire it is a zirconia, in which case the extra wire is purely and earth(grey), the other three being black signal, and two white heater wires.
These type work on 0.1V to 0.9V.
A titania sensor is used on much later fords normally amd work from 1V-0V or others are 5V to 0V.
It gets even more confusing now as the signal wire can now be blue as well as lots of other colours.
tabetha