I want to upgrade my alternator and I see it has FOUR wires.

The replacement I'm looking at is a 3-wire (1-wire optional)
from Powermaster, available at Jeg's for $119. See link:

http://www.jegs.com/i/Powermaster/713/8002/10002/-1

Question: What's the 4th wire?

Thanks in advance,
Rod in Virginia