It originally had a Cutout Relay, what youre calling a "2 Prong" That was NOT a Voltage Regulator
Its Possible to convert to what you call a "4 Prong" and if it has 4 terminals, its a Four Terminal VOLTAGE REGULATOR.
Originally the charge rate was manually controlled by a wire from the FLD Post on the Genny that wired to the Light Switch which had Low and High Charge settings
If you insist on changing it to a VR (Id leave it original if it were mine) you no longer use the wire from FLD on Genny up to the Light Switch but wire the genny and VR instead as follows:
BAT on VR to ammeters Load side terminal, thats the wire that used to wire to BAT on the Cutout Relay.....
FLD on VR to Gennys FLD Post, Do not run wire from FLD up to light switch any longer
GEN/ARM on VR to gennys ARM post
(L)OAD on VR feeds hot battery voltage to the light switch so loads like lights and ignition will work. The switch used to get its battery voltage off the ammeters Load side terminal but with the L VR terminal use it instead. NOTE the loads like lights and ignition would still work if you left them fed off the ammeter and didnt use the L on the VR
SURE YOU DONT WANT TO JUST BUY A "CUTOUT RELAY" (what you call 2 prong) and install its two wires same as old one and leave her original???
I have converted them and be sure the VR is suitable for your POS Ground