This book constitutes the thoroughly refereed post-proceedings of the 4th International Workshop on Declarative Agent Languages and Technologies, DALT 2006, held in Hakodate, Japan in May 2006 as an associated event of AAMAS 2006, the main international conference on autonomous agents and multi-agent systems. The 12 revised full papers presented together with 1 invited talk and 3 invited papers - fully revised to incorporate reviewers' comments and discussions at the workshop - were carefully selected for inclusion in the book. The papers combine declarative and formal approaches with engineering and technology aspects of agents and multiagent systems, thus gaining more and more attention in important application areas such as the semantic Web, Web services, security, and electronic contracting.