What is this 4-lead component that looks like enamelled wire?
Figure 1. A 4-terminal current shunt.

The highlighted item is a 4-terminal current shunt. The two terminals circled in red are the current terminals. The two circled in blue are the voltage measurement terminals.

This arrangement is known as the Kelvin (4-wire) Resistance Measurement technique. 



Figure 2. The Kelvin (4-wire) Resistance Measurement.

The principle of operation is that the voltmeter circuit draws such a little current that wire resistance does not induce any voltage drop. The voltage reading, therefore, is a true reading of the voltage across the shunt resistor.

On the circuit pictured note that the voltage readings are taken inside the current circuit soldered joints so they and the loops of wire to the actual shunt do not affect the reading.