Schematic figure of a four-terminal thermoelectric heat engine. The QDs system is connected to a resistor circuit. The four resistors (denoted as R_i, i = 1,2,3,4) are connected to the four electrodes. The electrochemical potential and the temperature of the electrode i are μ_i and T_i, respectively. J_i (i = 1,2,3,4) represents the current flowing through the resistor R_i.