Experimental studies on the metastable solubilities and the physicochemical properties (density, refractive index, conductivity, and pH value) in the aqueous reciprocal quaternary system (NaCl + MgCl2 + Na2SO4 + MgSO4 + H2O) at 273.15 K were determined with the isothermal evaporation method. According to the experimental results, the dry-salt-phase diagram, water-phase diagram, and physicochemical properties versus composition diagrams were plotted. It was found that there are two three-salt cosaturated points, five metastable solubility isotherm curves, and four metastable crystallization fields corresponding to sodium chloride, epsomite, mirabilite, and bischofite. Neither solid solutions nor double salts were found. On the basis of Pitzer's semiempirical equation for a mixed aqueous electrolytic solution, the extended HW model (Harvie and Meare) and the temperature-dependent equation were used for fitting the Pitzer single-salt parameters beta((0)), beta((1)), beta((2)), and C-phi for NaCl, MgCl2, Na2SO4, and MgSO4, the mixing ion-interaction parameters theta(Na,Mg), theta(Cl,SO4), Psi(Na,Mg,Cl), Psi(Na,Mg,SO4), Psi(Na,Cl),(SO4), and Psi(Mg,Cl,SO4), and the Debye-Huckel parameter A(phi) in the system at 273.15 K, which are not reported in the literature. In addition, the average equilibrium constants of metastable equilibrium solids at 273.15 K were obtained with the method of the activity product constant. The prediction of solubilities for the reciprocal quaternary system are then demonstrated. A comparison between the calculated and experimental results shows that the predictive solubilities obtained with the extended HW model agree well with experimental data.