This book constitutes the refereed proceedings of the 4th International Conference on GeoSpatial Semantics, GeoS 2011, held in Brest, France, in May 2011. The 13 papers presented together with 1 invited talk were carefully reviewed and selected from 23 submissions. The papers focus on formal and semantic approaches, time and activity-based patterns, ontologies, as well as quality, conflicts and semantic integration. They are organized in topical sections on ontologies and gazetteers, activity-based and temporal issues, models, quality and semantic similarities, and retrieval and discovery methods.