The experimental result on dielectric and mechanical properties of PbZrxTi1-xO3 driven by combined alternating electrical and mechanical loadings in various phase differences has been obtained by Zhou etal [J. Appl. Phys. 96, 6634 (2004)]. This paper presents the numerical simulation of this result using a two-dimensional four state Potts model. In this model, there are four different dipole orientations to reflect the coexistence of 90° and 180° domain walls. The coupling between the electrical and mechanical responses is implemented by the presence of two different ferroelastic strain states which are associated to the four different dipole orientations. The interactions of these dipoles and strains together with the mechanical energy density are incorporated into the system Hamiltonian. The enhanced and reduced electromechanical responses when the electric field and the compressive stress are out of phase and in phase, respectively, are reproduced by our model.