Arguments presented in the first three papers of this series (1, J. Phys. Chem. 1987, 91, 5818; 2, 1994, 98, 2222; 3, 1994, 98, 8591) supporting a "mixture model" of liquid water are extended to describe pressure effects. In the present paper, a quantitative assessment is given in terms of mixture model parameters of the temperature dependence of the isothermal compressibility of liquid water at atmospheric pressure. The "anomalous" minimum in this quantity near 50 degrees C is shown to emerge naturally.