Lead chalcogenide quaternary systems have been shown to provide high thermoelectric (TE) efficiency superior to those of binary and ternary lead chalcogenides, arising from both altered electronic band structures and a reduction in lattice thermal conductivity. Here, we have synthesized single-phase samples of the quaternary compound (PbTe)(0.55)(PbS)(0.1)(PbSe)(0.35) doped with Na and characterized their TE properties. We show that the dopant solubility is limited to 1 at. %. A very low lattice thermal conductivity of similar to 0.6 W m(-1) K-1 at 850 K is achieved at all dopant concentrations because of phonon scattering from point defects associated with solute atoms with high contrast atomic mass. As a result, a high TE figure of merit of approximately 1.5 is achieved at 823 K in heavily doped samples. Moreover, the figure of merit is greater than 1 over a wide temperature range above 675 K.