Paul,
I think you mean 4 volts. Amperes, or amps, is a unit of current. Voltage, or volts, is measured between the positive and negative terminals of a DC circuit, or, with AC, between hot and ground. If you are measuring the voltage between the grounded and hot rails and only get 4 volts with not load present i.e. train running, you have a very bad connection somewhere. I would suggest soldering small jumpers over the track joint.