This book constitutes the proceedings of the 4th International Conference on 6G for Future Wireless Networks, 6GN 2021, held in Huizhou, China, in October 2021. The 63 full papers were selected from 136 submissions and present the state of the art and practical applications of 6G technologies. The papers are arranged thematically in tracks as follows: Advanced Communication and Networking Technologies for 5G/6G Networks; Advanced Signal Processing Technologies for 5G/6G Networks; and Educational Changes in The Age of 5G/6G.