We propose a means of constructing simplified quaternary phase diagrams based on a thermodynamic approach using known and estimated data for Gibbs free energy of formation. Isothermal, isobaric sections of the condensed phase diagrams are built up as tie lines, tie planes, and tie tetrahedra (representing two‐, three‐, and four‐phase equilibrium, respectively) in a regular tetrahedron. This extends the now well‐established methodology for ternary systems described, for instance, by Beyers [J. Appl. Phys.56, 147 (1984); Mat. Res. Soc. Symp. Proc.47, 143 (1985)]. The procedure is illustrated by reference to Ti‐Si‐N‐O, Al‐Si‐N‐O, Ti‐Al‐Si‐N and Ti‐Al‐Si‐O, systems which are relevant to interactions occurring at various interfaces during the formation of contacts in integrated circuits. These phase diagrams are then used to predict the stability of—or reactions in—metallization layers and thin‐film systems involving four elements. In addition, a method is suggested to estimate unknown free energies of formation from observations of thin‐film reactions and stability.