Error estimates for the mixed finite element solution of4th order elliptic problems with variable coefficients, which, in the particular case of aniso-/ortho-/isotropic plate bending problems, gives a direct, simultaneous approximation to bending moment tensorfield $\Psi= (\psi_{ij})_{1 \le i,j \le 2}$ and displacement field 'u', have been developed considering the combined effect of boundary approximation and numerical integration.