This volume contains the proceedings of the 4th International Workshop on Critical Information Infrastructures Security (CRITIS 2009). The workshopwas held from September 30 to October 2 in the Gunnewig ] Hotel Bristol in Bonn, Germany. TheworkshopwasorganizedbytheFraunhoferInstituteforIntelligent Analysis and Information Systems (IAIS), Sankt Augustin, Germany. CRITIS 2009 continued the series of successful CRITIS Workshops. Com- nies, researchinstitutions, andgovernmentalorganizationsfromallmainareasof critical infrastructures took an active part in supporting CRITIS and we found CRITIS...