This book constitutes the thoroughly refereed post-proceedings of the 4th International Workshop on Engineering Self-Organising Applications, ESOA 2006, held in Hakodate, Japan in May 2006 as an associated event of AAMAS 2006, the 5th International Joint Conference on Autonomous Agents and Multi-Agent Systems.The 7 full papers presented together with 6 invited papers were carefully selected for inclusion in the book. The authors' revisions have been significantly improved by the reviewers' comments and the discussions following the presentation at the workshop. The papers are organized in topical sections on overall design and fundations, algorithms and techniques, applications, as well as self-organization and evolutionary computing.