The Fourth International Conference on the Nature and Ontology of Spacetime will commemorate the one hundredth anniversary of the publication of Einstein's general relativity in Annalen der Physik in 1916. While the Scientific Organizing Committee invites papers from physicists and philosophers on any topics related to the nature and ontology of spacetime, at this conference we especially wish to encourage submissions on not fully explored implications of general relativity for other areas of research in physics, cosmology, mathematics, philosophy of physics and for our understanding of the world. In addition to talks and poster sessions, special tea / coffee hours will be included in the program to stimulate more informal discussions among the participants. For this reason we encourage interested colleagues to attend the conference even if they do not plan to give a talk or present a poster. Contributed papers should be submitted in the form of extended abstracts of up to two pages by Sunday, January 31, 2016. Submissions will be reviewed by the Scientific Organizing Committee and notification of acceptance will be given by Monday, March 14, 2016. The extended abstracts, in English, should be emailed to firstname.lastname@example.org. The conference proceedings will be published.