This book constitutes the thoroughly refereed post-proceedings of the 4th International Workshop on Peer-to-Peer Systems, IPTPS 2005, held at Cornell University, Ithaca, NY, USA, in February 2005.

The 24 revised full papers were carefully selected during two rounds of reviewing and improvements from 123 submissions. The papers document the state of the art in peer-to-peer computing research. They are organized in topical sections on security and incentives, search, multicast, overlay algorithms, empirical studies, and network locality. The proceedings also include a report with a summary of discussions held at the workshop.