Why does the solenoid have 4 connections.
Two large for main start power.
One for solenoid power
And one other that goes to where?
Would love to know as now in the final week or so of rebuild.
1975 280SLC

Solenoid is another name for relay. Electricians have yet another name called "contactor". It would sure help people to understand if everybody used the same name!

Anyway search google.com for how a relay works and that should explain things. Basically a relay is an electrically operated switch. So there would be the two wires you are switching. And then the two wires to power the on/off switch.

Sometimes they use the metal automobile ground as one or two of those wires (- negative). But engines are dirty oily things, maybe they are running wires directly rather than using metal as a ground to be sure there are good connections?

Note the reason they would use a relay is that it is difficult to connect large wires to a key switch. Imagine running those large battery cables under the dash to a key switch!

Being you car is older, the small unused terminal is an ignition ballast resistor bypass. This terminal has power coming out if it only when you are cranking.

Battery voltage drops when cranking and the bypass assures hot spark. If you have electronic ignition, this terminal is generally not used but be sure to look at your specific cars wiring diagram.