The water activities of aqueous electrolyte mixtures of NaCl-KCl-NH4Cl-H2O were experimentally determined at T = 298.15 K by the hygrometric method at a total ionic strength from 0.4 mol (.) kg(-1) to about saturation of the solutes for different ionic strength fractions y of NaCl(aq) (y) I-NaCl/I-tot) of 1/3, 1/2, and 2/3 with the constant rapport molal ratio r of KCl(aq) on NH4Cl(aq) of unity (r) m(KCl)/m(NH4Cl)). The data allow the deduction of new osmotic coefficients. The results obtained were correlated using Pitzer's model and Dinane's mixing rules ECA I and ECA II for calculations of the water activity in mixed aqueous electrolytes. A new Dinane-Pitzer model is proposed for the calculation of osmotic coefficients in quaternary aqueous mixtures using the evaluated ternary and quaternary ionic mixing parameters obtained from the system studied. The solute activity coefficients of the components in the mixture were also determined for different ionic strength fractions y of NaCl(aq).