This book constitutes the refereed proceedings of the 4th International Workshop on Ambient Assisted Living, IWAAL 2012, held in Vitoria-Gasteiz, Spain, in December 2012. The 58 research papers were carefully reviewed and selected from various submissions. The papers are organized in topical sections such as intelligent healthcare and home-care environments, AAL environments, sensing and monitoring, human-computer interaction at assistive environments, semantic modeling for realizing AAL, and application domains.