The triple-differential cross section for electron impact ionization of the 4s orbital of krypton (binding energy 27.5 eV) has been measured in coplanar symmetric kinematics. The experimental results are presented for outgoing electron energies of 85, 50, 20, 10 and 4 eV. The results are compared with two distorted wave Born approximation calculations; the influence of post-collision interaction (PCI) on the cross sections is investigated by incorporating correct final state boundary conditions in one of the calculations. The influence of PCI is found to be very important at the lower energies. Inclusion of the three-body final state boundary conditions improves the agreement between theory and experiment, but there are still significant discrepancies, indicating that additional physical effects (such as higher-order interactions) are important at lower energies.