This book constitutes the proceedings of the 4^th International Workshop on Emerging Technologies for Authorization and Authentication, ETAA 2021, held in Darmstadt, Germany, on October 8, 2021. The workshop was co-located with ESORICS 2021. The 11 full papers presented in this volume were carefully reviewed and selected from 14 submissions. The workshop presents new techniques for biometric and behavioral based authentication, authentication and authorization in the IoT and in distributed systems in general, including smart home environment.