I got this used boost gauge with the harness and data link but as usually, no boost sensor. I noticed that the new gauges from GReddy has only 3 wires coming from the sensor but mine seems to have 4 wires.
What's the 4th wire for? I understand 3 wires but the purpose of the 4th one is a bit mystery.

Also, a stupid question, can I hook it up with the stock boost sensor(FD)?

 

If you buy an older electonic part without all the pieces, you're shooting yourself in the foot. Finding parts is damn near impossible.

You can't use the stock sensor - it's very likely too narrow range to make the boost gauge work properly. One of the newer Greddy boost sensors might work, but they're quite expensive new - over $100 just for the sensor, and you have to buy the harness separately.

Dale