(a) The maximum efficiency ηmax4T and (c) the efficiency at the maximum output power η^4T(W_max) for four-terminal thermoelectric devices as functions of the QD energy E₄ for different energies E₂. (b) and (d) present the ratio of the energy efficiency between the four-terminal and the three-terminal heat engines under the maximum energy efficiency and the maximum power conditions, respectively. The other parameters are E₁ = 1.0k_BT, E₃ = 3.0k_BT, μ = 0, and t = −0.2k_BT.