This book constitutes the carefully refereed and revised selected papers of the 4th Canada-France MITACS Workshop on Foundations and Practice of Security, FPS 2011, held in Paris, France, in May 2011. The book contains a revised version of 10 full papers, accompanied by 3 keynote addresses, 2 short papers, and 5 ongoing research reports. The papers were carefully reviewed and selected from 30 submissions. The topics covered are pervasive security and threshold cryptography, encryption, cryptanalysis and automatic verification, and formal methods in network security.