This book constitutes the refereed proceedings of the 4th German Conference on Multiagent Systems Technologies, MATES 2006, co-located with Net.ObjectDays (NoDe 2006). The 15 revised full papers presented were carefully reviewed and selected from 52 submissions. The papers are organized in topical sections on agent communication and interaction, applications and simulation, agent planning, agent-oriented software engineering, as well as trust and security.