This book constitutes refereed proceedings of the Fourth International Conference on Emerging Technology Trends in Electronics, Communication and Networking, ET2ECN 2021. The volume covers a wide range of topics, including electronic devices, VLSI design and fabrication, photo electronic systems and applications, integrated optics, embedded systems, wireless communication, optical communication, free-space optics, signal processing, image/audio/video processing, wireless sensor networks, next-generation networks, network security, and many others. The book will serve as a valuable reference resource for academia and researchers across the globe.