this book constitutes the refereed proceedings of the 4th asian symposium on programming languages and systems, aplas 2006, held in sydney, australia in november 2006. the 22 revised full papers presented together with 2 invited talks and 1 tutorial were carefully reviewed and selected from 70 submissions. the symposium is devoted to both foundational and practical issues in programming languages and systems. among the topics covered are semantics, logics, type theory, language design, program analysis, optimization and transformation, software security, verification, compiler systems, interpreters, abstract machines, domain-specific languages and systems, as well as programming tools and environments.