This book constitutes the refereed proceedings of the 4th International Symposium on Location and Context Awareness, LoCA 2009, held in Tokyo, Japan, in May 2009. The 18 revised full papers presented were carefully selected from 77 submissions. The papers describe original, previously unpublished research results, including new hardware platforms for sensing location and context, machine learning techniques for inferring user location and context from low-level sensor data, location and context representation, management, and distribution, and privacy policies and communication protocols for location and context information.