Polycrystalline Al alloys are known to deform inhomogeneously. Characterization of this process in 3D is crucial for understanding deformation and failure mechanisms, and hence for improving the design of Al alloys. In this work, the local strain heterogeneities in a particle-containing Al alloy have been investigated by coupling a 4D experiment with 4D simulations. Firstly, multimodal X-ray tomography, i.e. attenuation tomography and laboratory-based diffraction contrast tomography, was applied to characterize the microstructure and to follow the local plastic deformation of a fully recrystallized Al-4mass%Cu alloy in 3D during an in-situ tensile test. Based on the experimental data, several correlations were examined between the 3D grain structure and the local plastic strains determined using a microstructural feature tracking method before and after tensile deformation. Interestingly, only a very weak correlation between the intragranular strain variation and the grain size was found, and no correlation between strain and grain orientation was observed. Secondly, the 3D grain structure was utilized as input in crystal plasticity finite element modeling, to reproduce digitally the heterogeneous plastic deformation of the mapped sample volume. By analyzing the observed significant discrepancies between the 4D experimental results with 4D simulations, new insight into the heterogeneous plastic deformation process is obtained. Future improvements in experimental characterization and modeling points are finally suggested.