This book constitutes the refereed proceedings of the 4th International Conference on Web Engineering, ICWE 2004, held in Munich, Germany, in July 2004. The 25 revised full papers, 42 short papers, and 24 posters presented together with abstracts of 3 invited papers were carefully reviewed and selected from 204 submissions. The papers are organized in topical sections on Web quality and usability, conceptual modeling; Web services and distributed processes and systems; Web metrics, cost estimation, and measurement; personalization and adaptation of Web applications; code generation and tools; development process and process improvement of Web applications; semantic Web applications; performance; Web data models, query, and representation languages; Web interface engineering; security, safety, and reliability; Web mining, user models, and data analysis.