The last segment of a four-part investigation on the influence of phosphorus and silicon on neutron-induced void swelling of austenitic stainless steels has been completed. This final segment explored the influence of these elements on the macroscopic swelling of developmental alloys irradiated in the 20% cold-worked condition in the Fast Flux Test Facility. As was also seen in the previous three series, a non-monotonic swelling response with respect to phosphorus concentration is sometimes observed. Therefore it is not safe to assume that phosphorus and/or silicon additions will always reduce swelling.