The Reasoning Web summer school series is a well-established event, attracting experts from academia and industry as well as PhD students interested in fo- dational and applicational aspects of the Semantic Web. This volume contains thelecturenotesofthefourthsummerschool, which took place in Venice, Italy, in September 2008. This year, the school focussed on a number of important application domains, in which semantic web techniques have proved to be p- ticularly e?ective or promising in tackling problems. The ?rst three chapters provide introductory material to: - languages, formalisms, and standards adopted to encode semantic information; - "soft" extensions that might be useful in contexts such as multimedia or social network applications; - controlled natural language techniques to bring ontology authoring closer to end users. The remaining chapters cover major application areas such as social networks, semantic multimedia indexing and retrieval, bioinformatics, and semantic web services. Thepresentationshighlightedwhichtechniquesarealreadybeingsuccessfully applied for purposes such as improving the performance of information retrieval algorithms,enablingtheinteroperationofheterogeneousagents,modellinguser's pro?les and social relations, and standardizing and improving the accuracy of very large and dynamic scienti?c databases. Furthermore, the lectures pointed out which aspects are still waiting for a solution,andthepossiblerolethatsemantictechniquesmayplay,especiallythose reasoningmethodsthathavenotyetbeenexploitedtotheirfullpotential.Wehope thatthe school'smaterialwillinspire further exciting researchinthese areas. We are grateful to all the lecturers and their co-authors for their excellent contributions, to the Reasoning Web School Board, and Norbert Eisinger in particular, who helped in several critical phases, and to the organizations that supported this event: the University of Padua, the MOST project, and the N- work of Excellence REWERSE.