This book constitutes the refereed proceedings of the 4th International Workshop on Real-World Wireless Sensor Networks, REALWSN 2010, held in Colombo, Sri Lanka, in December 2010.The 11 full papers and the 5 short papers presented were carefully reviewed and selected from 34 submissions. The papers are organized in topical sections on applications; OS support and programming; communication & MAC; and poster and demonstration abstracts.