The wire medium consisting of an array of parallel thin metallic wires was previously studied by using an effective medium with spatial dispersion. In this paper, the validity of conventional effective model was examined analytically and numerically by studying a canonical structure of the wire medium. It is noted that the conventional model fails for high transversal spatial harmonics, which consequently results in discrepancy in the scattering between the effective model and the physical structure. In this study, we propose a new effective model to include higher order spatial dispersions: instead of the second-order expansion, the proposed dispersion equation is based on the fourth-order expansion of the dispersion equation of the photonic states. Compared with the 3D full-wave simulation results of the wire medium, the proposed model has demonstrated significant improvement in numerical accuracy in characterizing the EM behavior in this type of metamaterials.