Dynamic radiation therapy techniques require that the treatment head setup varies in time and space during dose delivery. Examples are leaf motion in intensity modulated x-ray therapy, modulator wheel rotation in conventional proton therapy and variable magnet settings in proton beam scanning. Consequently, for Monte Carlo dose calculation the results of several independent simulations usually have to be combined. Depending on the complexity and the required accuracy this can become cumbersome. We present a technique to simulate time-dependent geometries within a single four-dimensional Monte Carlo simulation using the GEANT4 Monte Carlo package. Results for proton therapy applications are shown.