Here are the refereed proceedings of the 4th International Conference on Adaptive Hypermedia and Adaptive Web-Based Systems, AH 2006, held in Dublin, Ireland, June 2006. The book presents 22 revised full papers and 19 revised short papers together with abstracts of 3 keynotes, 12 poster papers, and 14 doctoral consortium posters. Topics include pioneering theories, techniques, and innovative technologies to provide dynamic personalization, adaptation, and contextualization of hypermedia resources and services.