Edited version!
The 8 Volts is a large drop, so could do with seeing your battery terminal voltage at the same time as heater plugs powered up.
Going on what you have said above, my theory: you'll have around 12 volts at the battery, when you have 8 volts at your glow plugs.
I believe your heater plug relay is fubar or you have a poor high resistance connection in that circuit, so basically your heater plugs are not getting hot.
( I say this because, If the relay contact has high resistance, the relay contact will not pass enough current and you will get a voltage drop. (but will show 12v plus when not connected/underload))
One other test to try, remove the common +12v from the plugs and test resistance of each plug.
Additionally, you will suss out if one or more heater plug is failing if a plug has much higher resistance. In my limited experience the general resistance of a heater plug is <0.5 ohms, I've not seen one above 1.0 ohm unless its failing. However see what all four are reading, from that it will show up any variance. You could also see what voltage your getting on the +12V common disconnected from the plugs, and then connect one plug at a time and see what voltage is present as you go on, this will prove if a plug is dragging the circuit down. Sound a bit laborious? it is easier with a friend doing ignition on/off.
When I got my Pathfinder with 16k miles on it, if I (usually my wife) started it from cold without waiting for the glow plug light to go out, the engine smokes a short while.
Hope all the above makes sense.
Moderators please delete my post before this one, Thank you
