This book constitutes the refereed proceedings of the 4th Usability Symposium of the Human-Computer Interaction and Usability Engineering Workgroup of the Austrian Computer Society, USAB 2008, held in Graz, Austria, in November 2008. The 7 revised full papers and 26 revised short papers presented were carefully reviewed and selected from 85 submissions. The papers are organized in topical sections on cognitive processes and end user experience, social software and collaborative knowledge development, knowledge processing, assessment and human performance, human centered development and design accessibility, user centred information visualization and multimedia, adaptivity and personalization in ubiquitous learning systems, and human centred design for safety critical systems.