This volume constitutes the revised papers of the 4th European Conference on Smart Sensing and Context, Euro SSC 2009, held in Guilford, UK, in September 2009. This volume consists of 16 full papers. Each paper received at least three peer reviews. The conference and proceedings were structured into 6 main tracks which discussed the key themes addressed by EuroSCC 2009: activity recognition, information aspects of context-aware sensor and actuator systems, context-aware service platforms, context processing, reasoning and fusion, real-world experiences with deployed systems, and context-aware frameworks in mobile environments.