These proceedings record the papers presented at the 4th International Conf- ence of B and Z Users (ZB 2005), held in the city of Guildford in the south-east ofEngland. Thisconferencebuiltonthesuccessofthepreviousthreeconferences in this series, ZB 2000, held at the University of York in the UK, ZB 2002, held at theLaboratoire Logiciels Sys&tgrave; emes R´ eseaux within theInstitut d'Informatique et Math´ ematique Appliqu´ ees de Grenoble (LSR-IMAG) in Grenoble, France, and ? ZB 2003, held in Turku in Finland hosted by Abo Akademi University and the TurkuCentreforComputerScience(TUCS). ZB2005washeldattheUniversity of Surrey, Guildford, UK, hosted by the Department of Computing. The U- versity has always placed particular emphasis on the applicability of its research and its relationship with industrial partners. In this context it is building up its formal methods activity as an area of strategic importance, with the establi- ment of a new group within the Department of Computing, and also with its support for this conference. B and Z are two important formal methods that share a common conceptual origin; they are leading approaches in industry and academia for the speci?- tion and development (using formal re?nement) of computer-based systems. At ZB 2005 the B and Z communities met once again to hold a fourth joint c- ference that simultaneously incorporated the 15th International Z User Meeting and the 6th International Conference on the B Method.