A finite integrationmethod on a four-dimensional space-time grid is studied for the computation of electromagnetic wave propagation, where a non-uniform time-step distribution is naturally introduced. A dual grid based on the Hodge duality and the Lorentz metric is proposed to provide a simple constitutive equation for electromagnetic variables. An explicit time-marching scheme for a non-uniform space-time grid achieves a more efficient electromagnetic field computation than the conventional FDTD method.