First off, If you have a gauge drawing 4 amps or above...toss it it's way past shot..more like a few mil-amps would be more like it..
Without a LOAD on a resistor..It will show the same voltage in as it does out..Because it is doing no useful work. It is just a straight wire..Current is used in the resistor and changed to heat when a load (the gauge, a light, ect..) is imposed on the resistor..current goes high..voltage goes low. Tell your Guy..whoever told you you need 4 of them..he needs to study OHMS law..sounds like at 12 bucks a pop..he's studied financial management real well...
Current = 12 volts divided by 1.5 ohms = 8 amps
To determine the resistance of your gauge, get out your DVOM, set it for OHMS scale, R X 1 or autorange, calibrated for 000, disconnect ALL the gauge wires..measure across the power and ground of the gauge..Whatever that reading may be..say 90 ohms..apply a little more ohms law..
Current=6 Volts divided by 90 ohms = 0.666 amps..(about 1/2 an amp)
You say something is burning up ? what is burning up?
Sounds to me like you don't have a drop problem at all..but a sender wire open or shorted to ground Or a bad sender, Or a mis matched sender.
The easiest thing to do (and a heck of a lot cheaper..)get a 6 volt lantern battery, remove power and ground from the gauge, and jump in the lantern battery..If the gauge pegs one way or the other..the sender wire is open or short, Or the sender is wrong for the gauge. Troubleshoot that first , then go back to a dropping resistor from the 12 volt system.
You may have another remote problem..If the car was 6 volt positive ground, and you didn't rewire the gauge..your putting the resistor between the gauge and ground making the gauge a dropping resistor for the resisror..(it's dumping the load on the gauge) ..
You need to strip those wires and rewire it for negative ground with he resistor feeding the + side of the gauge.be sure the - goes to the body (not power) and the + side goes to the resistor.