Thermoelectric power generation is an energy conversion technology from heat to electric energy, which can be applied to waste heat power conversion. Among thermoelectric materials (TE), PbTe-PbSe-PbS quaternary alloys and composites are promising candidates for thermoelectric power generation applications in the mid-temperature operating range from 500 to ~850 K. Besides, the thermoelectric performance of quaternary alloys and composites is not fully optimized regarding its composition and synthesis process. In the quaternary system, PbTe-PbSe-PbS, it was found that PbS will form nanoprecipitation in the matrix of quaternary alloy for a small content of PbS (≤0.07), which reduces the lattice thermal conductivity. The power factor of PbTe-PbSe-PbS quaternary alloys can be significantly enhanced by using a band convergence in PbTe1−xSex. The band structure modifications, with the result of simultaneous PbS nanoprecipitation, give rise to a high Z T value of 2.3 at 800 K for (PbTe)0.95−x(PbSe)x(PbS)0.05. The chemical potential tuning by effective K-doping ( x = 0.02) and PbS substitution reveals a high power factor and low thermal conductivity, resulting in a comparatively high Z T value of 1.72 at 800 K. The combination of a high Seebeck coefficient and low thermal conductivity results in a very high Z T value of 1.52 at 700 K as n-type materials for low Cl-doped ( x = 0.0005) (PbTe0.93−xSe0.07Clx)0.93(PbS)0.07 composites. Therefore, this review presents the simultaneous emergence of effective chemical potential tuning, band convergence, and nanoprecipitation, giving rise to a significant enhancement of the thermoelectric performance of both p - and n -type PbTe-PbSe-PbS quaternary alloy and composite TE materials.