The present research demonstrates a facile one-pot heating process without injection to synthesize an important light harvesting quaternary nanocrystal: wurtzite copper–zinc–tin sulfide (w-CZTS). High quality w-CZTS nanocrystals can be easily obtained by mixing all the precursors and simply heating to the reaction temperature. The nano-crystal formation mechanism is thoroughly investigated and resolved by X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). It starts with the nucleation of plasmonic djurleite Cu_1.94S, subsequent growth of CZTS–Cu_1.94S heterostructures and inter-diffusion of cations and then finally leads to single phase and single crystal w-CZTS nanocrystals. The mechanism of nanocrystal formation can be applied universally regardless of the type of zinc and tin precursor for high quality w-CZTS nanocrystals. The in-depth interpretations of the reaction mechanism of this process significantly advance the current knowledge of multi-component nanocrystal formation. The developed method is scalable for high throughput and low cost w-CZTS suspensions which await practical photovoltaic applications.