The four-body model has been used to calculate the fully differential cross-sections (FDCS) for the single ionization of helium by 100 MeV/amu C6+
impact in geometries. By compar-ing with experimental data and other theories, we find the results of four-body model are in very good agreement in the scattering plane, but poor agreement out of the scattering plane. Accordingly, the contributions of different scattering amplitudes to FDCS are analyzed. It is found that the cross sections due to the interference of the scattering amplitudes between projectile-target nucleus interaction and projectile-ejected electron interaction almost tend to experimental results around the recoil region in geometries. In particular in the perpen-dicular plane, the cross section originating from interference of the scattering amplitudes between projectile-target nucleus and projectile-ejected electron interactions yields an ex-perimental double-peak structure in the angular distribution. However, this feature could not be presented by the interference of the three amplitudes. Thus, the failure of the four-body model predicting the feature in this geometry may be attributed to an inappropriate weighting of the three amplitudes.