We could not find any detailed study of mixtures of four weak species in the literature and it turns out that the tools proposed here allow studying this case, provided that we restrict ourselves to "nice" combinations of parameters. Indeed, the four acids mixture represents an interesting and still somewhat simple situation under the following conditions: first, the contribution of H 3 O + and OH -is neglected and, second, we consider a highly symmetrical set of concentrations and pK a 's in the sense defined hereafter.