We have developed simulation codes to handle electromagnetic process in Geant4 in an attempt to improve the accuracy of dose calculations for ion radiotherapy. Geant4 is a Monte Carlo simulation toolkit for simulating the passage of particles through matter, and capable of handling all physics processes including electromagnetic, hadronic and nucleus-nucleus interactions which are indispensable to calculate three-dimensional dose distributions in ion therapy. In this work we report the recent progress for Geant4 models for ions and new validation results, which were obtained for depth-dose distributions of carbon ions incident on water against experimental measurements. We found simulations to reproduce the experimental data fairly well.