Adsorption equations are developed for the four layer model assuming that the anions and cations of the basic electrolyte are adsorbed in two distinct layers, located at different distances from the surface. The theoretical development has been based on the ideas first launched by Yates and Chan, and then elaborated in detail by Leckie and Morel, for the case of the triple layer model. Among various advantages of our new theoretical approach, the continuity of the electrical capacitances as pH functions is a very essential one. The developed expressions predict correctly what is found in the experiment, i.e., that the electrical capacitancies depend on the concentration of the basic electrolyte. Our new theoretical approach predicts the difference between PZC and IEP values to he one of the fundamental features of these adsorption systems. This new theoretical approach leads to adsorption equations which are almost as simple and easy to use as those developed for the popular triple layer model.