This book is an entirely post-proceedings referee the 4th International Workshop on Declarative Agent Languages and Technologies, DALT 2006, held in Hakodate, Japan in May 2006 as an associated event 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 - completely revised to incorporate comments reviews and discussions at the workshop - carefully selected for inclusion in this book.