Directional crystallization and thermal analysis were used to construct a nontrivial polythermal section of the solid–liquid diagram of the Cu–Fe–Ni–S system. We have carried out directional crystallization of melt of the following composition (in mol%): Fe 32.06, Ni 1.70, Cu 15.80 and S 50.4. Results of microscopic and electron microprobe analyses showed that the grown ingot consists of five zones with different chemical and phase compositions. They formed during sequential crystallization of four phases from the following melt: monosulfide solid solution mss, non-stoichiometric isocubanite icb* and two intermediate solid solutions iss1 and iss2 with a similar composition. It was found that the chemical composition of the phases and the melt changes during crystallization. Thermal analysis of specially synthesized samples along the crystallization path was performed. The position of the liquidus and solidus in the 4D space was determined (T, xNi, xCu, xFe). These data were used for constructing the polythermal sections of the phase diagram along the crystallization path.