Hi Grif. The exciter wire from the firewall block and the cab originates at the key switch on trucks with the gauge dashes and at the charging light on trucks with the idiot light dashes. Either way the wire comes through the fire wall connector as a 16 gauge brown wire and goes to the voltage regulator on one of the end terminals.
It should have zero voltage on it with the key off and 12 volts with the key on. Measure it with a voltmeter and if the white wire is the exciter wire, it should show those readings. I have not seen any of the four wires jumpered to any other terminal on the regulator unless it is some special type of regulator I'm not aware of.
Here's a diagram showing what I have said.
As you can see the wire to no. 4 on the regulator is the exciter wire and it exits the regulator at terminal 2 and goes to the R terminal on the alternator.
The wire on terminal 1 goes to the alternator to the F terminal and was a blue wire on the original harness. If the F and R wires are switched at the alternator or the regulator, The blue wire which has 12 volts on it, will drain through the alternator diodes to ground and discharge the battery.
This is the plug in your alternator .
The black wire is a ground and should also be connected to the regulator which should be grounded to the radiator core support.
Hope this helps.