We present a new version of our optical model for solar cell simulation: GenPro4. Its working principles are briefly explained. The model is suitable for quickly and accurately simulating a wide range of wafer-based and thin-film solar cells. Especially adjusting layer thicknesses to match the currents in multijunction devices can be done with a minimum of computational cost. To illustrate this, a triple junction thin-film silicon solar cell is simulated. The simulation results show very good agreement with external quantum efficiency measurements. The application of an MgF₂ antireflective coating or an antireflective foil with pyramid texture is considered. Their effects on the implied photocurrents of top, middle, and bottom cells are investigated in detail.