Constitutes the refereed post-proceedings of the 4th International Workshop on Agents and Peer-to-Peer Computing, AP2PC 2005, held in Utrecht, Netherlands, on July 25th, 2005, in the context of the 4th International Joint Conference on Autonomous Agents and Multi-Agent Systems, AAMAS 2005.

Trust and Reputation.- Optimizing an Incentives’ Mechanism for Truthful Feedback in Virtual Communities.- A New View on Normativeness in Distributed Reputation Systems.- A Trust Management Scheme in Structured P2P Systems.- Incentive-Compatibility in a Distributed Autonomous Currency System.- Handling Free Riders in Peer-to-Peer Systems.- P2P Infrastructure.- Highly Available DHTs: Keeping Data Consistency After Updates.- Caching Indices for Efficient Lookup in Structured Overlay Networks.- Semantic Infrastructure.- A Semantic Marketplace of Negotiating Agents.- Semantic Web Service Composition Through a P2P-Based Multi-agent Environment.- Community and Mobile Applications.- A Low-Latency Peer-to-Peer Approach for Massively Multiplayer Games.- An Agent-Based Collaborative Framework for Mobile P2P Applications.- ACP2P: Agent-Community-Based Peer-to-Peer Information Retrieval – An Evaluation.- A Peer Ubiquitous Multi-agent Framework for Providing Nomadic Users with Adapted Information.