Liquid-liquid equilibrium (LLE) and solubility for the quaternary acetic acid-n-propyl alcohol-n-propyl acetate-water were carried out at 323.15 and 333.15 K (atmospheric pressure). A brief review of literature data is provided. The compositions of LLE were determined by gas chromatography analysis. Solubility data and critical points were obtained by the "cloud-point"technique method. The obtained data gave an opportunity to present the disposition of the critical curve formed by critical points and binodal surface in a regular composition tetrahedron. A set of critical curves led to a critical polythermal surface in the 3D concentration space. A detailed graphical analysis of data comparison is presented for binary and ternary subsystems. LLE modeling using the NRTL model shows that calculated data are in good agreement with the experimental results.