This book constitutes the thoroughly refereed post-proceedings of the 4th International Workshop on Agents and Peer-to-Peer Computing, AP2PC 2005, held in Utrecht, Netherlands, July 2005, in the context of the 4th International Joint Conference on Autonomous Agents and Multi-Agent Systems, AAMAS 2005. The 13 revised full papers cover trust and reputation, P2P infrastructure, semantic infrastructure, as well as community and mobile applications.