This book constitutes the refereed proceedings of the 4th International Conference on Ubiquitous Communications and Network Computing, UBICNET 2021, held in March 2021. Due to COVID-19 pandemic the conference was held virtually. The 17 full papers were selected from 59 submissions and are basically arranged in different sessions on 5G networks, millimeter wave communication systems and emerging applications; quantum communication, IoT and emerging applications; data analytics and cloud computing; artificial neural network, machine learning and emerging applications.