These sketches may help you: Alternator Regulator Wiring Diagrams
Our old alternator was case grounded so the ground wire didn't show on the first sketch. The following diagrams on that link show the connections for our external regulator, a Balmar MC612. All the wiring is identified.
Unless you want to buy a book, I recommend visiting Balmar's excellent website Welcome to Balmar DC Charging Solutions
. Look at their manuals for regulators and they'll show the wiring.
Since two of the four wires are pretty clear (red output and black ground) the others are for the ignition, field and tach.
The Balmar manuals should better familiarize you with the uses.
Maine Sail's question is very apt.