Annotation. This book constitutes the refereed proceedings of the 4th InternationalConference on Web Reasoning and Rule Systems, RR 2010, held inBrixen-Bressanone, Italy, in September 2010. The 9 revised full papers, 6 revised short papers, and 4 poster paperspresented together with 1 PhD paper, 2 system descriptions and 3 invitedpapers were carefully reviewed and selected from 41 submissions. Thepapers address all current topics in Web reasoning, Web-based knowledge, and rule systems such as representation techniques; rules andontologies; reasoning languages; efficiency and benchmarking, ; ontologylanguages; querying and optimization; reasoning with uncertainty, underinconsistency, and with constraints; rule languages and systems; ruleinterchange formats and markup languages; scalability; approximatereasoning; statistical methods and symbolic reasoning; as well assemantic Web services modeling and applications.