Welcome to the proceedings of REALWSN 2010, the 4th Workshop on Real- World Wireless Sensor Networks! After threemeetings in Europewedecided to holdREALWSN in exciting Sri Lanka. We want to thank the local organizers as well as the authors, attendees and members of the technical ProgramCommittee, Demo and Poster Chairs for making this event possible. As the name of the workshop suggests, REALWSN is a forum for people interested in real-world issues in the fascinating research area of wireless sensor networks.Despite many yearsofresearchthe deploymentof realsensornetworks isstillachallengingtask.Thebehaviorofrealdeployednetworksdi?erssubst- tially from the behavior of the same network in a simulator. The main objective of REALWSN is to bring together researchers and practitioners to understand these di?erences and boost the state of the art in this exciting ?eld. This year the program consisted of 11 full papers and ?ve short papers ca- fullyselectedfromover34submissions.SinceREALWSN2010wasastand-alone two-day event, the attendees could also look forward to a poster and demo s- sion with more than 10 contributions. The technical program covered topics from low-level communication and software development to a variety of re- worldsensor networkapplications,some of them tailoredto Asian wildlife which we think is particularly interesting. Thanks again to all people who contributed to the workshop: the Technical ProgramCommittee,thedemoandposterchairsKameswariChebroluandAdam Dunkels, the Publication Chair Luca Mottola and our sponsors that include the UniversityofColombo,theUppsalaVINNExcellenceCenterforWirelessSensor NetworksWISENET,InterBlocksLtd.andtheSustainableComputingResearch Group at the University of Colombo. The local Organizing Committee provided tremendous help that made REALWSN possible.