In this paper, we present S4D, a new open-source Python toolkit dedicated to speaker diarization. S4D provides various state-of-the-art components and the possibility to easily develop end-to-end diarization prototype systems. S4D offers a large panel of clustering, segmentation, scoring and visualization algorithms. S4D has been thought to be easily understood, installed, modified and used in order to allow fast transfers of diarization technologies to industry and facilitate development of new approaches. Examples, benchmarks on standard tasks and tutorials are provided in this paper. S4D is an extension of the open-source toolkit for speaker recognition: SIDEKIT.