This book constitutes the refereed proceedings of the 4th Annual International Conference on Wireless Algorithms, Systems, and Applications, WASA 2009, held in Boston, MA, USA, in August 2009. The 36 revised full papers presented together with 15 invited papers and 7 workshop papers were carefully reviewed and selected from numerous submissions. Providing a forum for researchers and practitioners worldwide, the papers address current research and development efforts of various issues in the area of algorithms, systems and applications for current and next generation infrastructure and wireless networks. They are divided in topical sections on applications, experimentation, power management; coverage, detection, and topology control; routing, querying, and data collection; localization, security, and services; scheduling and resource management; and online social networks, applications, and systems.