This volume constitutes the proceedings of the 4th International Conference on Frontiers in Cyber Security, FCS 2021, held in Haikou, China, in December 2021. The 20 full papers along with the 2 short papers presented were carefully reviewed and selected from 58 submissions. The papers are organized in topical sections on: intelligent security; system security; network security; multimedia security; privacy, risk and trust; data and application security.