This book constitutes the refereed proceedings of the 4th International Symposium on Ubiquitous Computing Systems, UCS 2007, held in Tokyo, Japan, in November 2007. The 16 revised full papers and eight revised short papers presented were carefully reviewed and selected from 96 submissions. The papers are organized in topical sections on security and privacy, context awareness, sensing systems and sensor network, middleware, modeling and social aspects, smart devices, and network.