We have used the dielectric-continuum model to study the surface and interface optical-phonon modes in a four-layer heterostructure with finite thickness. The eige-nvectors, the dispersion relation and the interface charge densities are obtained. All the previous results for a dielectric slab, bilayer, trilayer and infinite four-layer he-terostructures can be reded-iced from our general relation for a finite four-layer he-terostructure. The dispersion relations and their numerical illustration for some inte-resting and practical structures such as a finite step-quantum-well are also given. This work can be regarded as a generalization of the formalism in reference 8.