This book constitutes the thoroughly refereed post-proceedings of the 4th International Workshop on Privacy Enhancing Technologies, PET 2004, held in Toronto, Canada in May 2004.

The 20 revised full papers presented were carefully selected from 68 submissions during two rounds of reviewing and improvement. Among the topics addressed are mix networks, mix firewalls, traffic analysis, sequence analysis, re-encryption, electromagnetic eavesdropping, anonymity of banknotes, IPv6, trustworthy environments, flow correlation attacks, anonymity measuring, mix cascades, trust negotiation,...