A method of studying of the liquid liquid liquid equilibrium (LLLE) region in quaternary systems was proposed. It includes four basic stages: the study of the tetrahedron scan; the determination of a composition of coexisting liquid phases in ternary constituents; the choice of conjugate composition and analysis of compositions of three liquid phases at adding conjugate composition; the prediction of the type of LLLE splitting area in a composition tetrahedron. Different ways of conjugate composition adding were proposed for different types of the splitting diagram. Five quaternary systems with different types of three liquid phase region were studied using mathematical modeling and nonrandom two-liquid model. The structure of the three-phase immiscibility region was predicted for all systems. It was found that the prediction results and experimental and calculated data are in a good agreement.