We have applied our model concerning the phase structures of multiphase polymer blends to quaternary blends of polyethylene, polypropylene, polystyrene and poly(methyl methacrylate) of different overall compositions. Predictions of the phase morphologies of such multicomponent blends are based on minimization of the total interfacial free energy of the system. We compare predictions of the model with experimental observations, and show agreement in every case between the predicted and experimental morphologies. Furthermore, we show that the phase structure of a quaternary blend can be converted from one type of phase morphology to another, e.g. from an encapsulation type to the dispersion type, by using an interfacially active block copolymer. This shows the possibility of adjusting the phase morphologies of multicomponent systems as desired.