This book constitutes the refereed proceedings of the 4th International§Conference on Web Reasoning and Rule Systems, RR 2010, held in§Brixen-Bressanone, Italy, in September 2010.§The 9 revised full papers, 6 revised short papers, and 4 poster papers§presented together with 1 PhD paper, 2 system descriptions and 3 invited§papers were carefully reviewed and selected from 41 submissions. The§papers address all current topics in Web reasoning, Web-based knowledge,§and rule systems such as representation techniques; rules and§ontologies; reasoning languages; efficiency and benchmarking,; ontology§languages; querying and optimization; reasoning with uncertainty, under§inconsistency, and with constraints; rule languages and systems; rule§interchange formats and markup languages; scalability; approximate§reasoning; statistical methods and symbolic reasoning; as well as§semantic Web services modeling and applications.