Oxygen sensor on '04 Hemi

got some questions?

I got a fuel to air mixture guage and it says to hook it up to the upstream o2 sensor.
You install it to the data line to the PCM.
the o2 sensor has 4 wires on it, a garage said the data wire will have like 4 volts to it.
Can anyone tell me the right wire to use?
the colors are 2 white , grey , and black
Can anyone tell what all the wires do?
thanks for your help