If the cutout/regulator has 4 terminals, it's a regulator. And from your description (low charge rate, generator can't keep the lights and an open terminal on the VR) says the VR is wired wrong.
The regulator's BAT terminal should connect to the ammeter. And the regulator L terminal should go to the tractor's electrical loads ("hot" supply for both the ignition and light switches).
Punch the link below for a diagram.