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 Syst` emes R¿ eseaux within theInstitut d¿Informatique et Math¿ ematique Appliqü 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.

Specification Before Satisfaction: The Case for Research into Obtaining the Right Specification -Extended Abstract-.- Visualising Larger State Spaces in Pro B.- Non-atomic Refinement in Z and CSP.- Process Refinement in B.- CZT: A Framework for Z Tools.- Model Checking Z Specifications Using SAL.- Proving Properties of Stateflow Models Using ISO Standard Z and CADiZ.- A Stepwise Development of the Peterson's Mutual Exclusion Algorithm Using B Abstract Systems.- An Extension of Event B for Developing Grid Systems.- The Challenge of Probabilistic Event B-Extended Abstract-.- Requirements as Conjectures: Intuitive DVD Menu Navigation.- A Prospective-Value Semantics for the GSL.- Retrenchment and the B-Toolkit.- Refinement and Reachability in Event_B.- A Rigorous Foundation for Pattern-Based Design Models.- An Object-Oriented Structuring for Z Based on Views.- Component Reuse in B Using ACL2.- GeneSyst: A Tool to Reason About Behavioral Aspects of B Event Specifications. Application to Security Properties.- Formal Verification of a Type Flaw Attack on a Security Protocol Using Object-Z.- Using B as a High Level Programming Language in an Industrial Project: Roissy VAL.- Development via Refinement in Probabilistic B - Foundation and Case Study.- Formal Program Development with Approximations.- Practical Data Refinement for the Z Schema Calculus.- Slicing Object-Z Specifications for Verification.- Checking JML Specifications with B Machines.- Including Design Guidelines in the Formal Specification of Interfaces in Z.- Some Guidelines for Formal Development of Web-Based Applications in B-Method.