WYK: Mobile Device Authentication Using the User's Address Book -- Future-Proof Web Authentication: Bring Your Own FIDO2 Extensions -- Heartbeat-based authentication on smartwatches in various usage contexts Hazel Murray and David Malone -- Quantum multi-factor authentication -- An Interface between Legacy and Modern Mobile Devices for Digital Identity -- Facial Recognition for Remote Electronic Voting Missing Piece of the Puzzle or Yet Another Liability? -- Integrating a Pentesting Tool for IdM Protocols in a Continuous Delivery Pipeline -- Mimicry Attacks Against Behavioral-based User Authentication for Human-Robot Interaction -- Private Data Harvesting on Alexa using Third-Party Skills -- Trust Aware Continuous Authorization -- Ask App Not to Track": The Effect of Opt-in Tracking Authorization on Mobile Privacy.

This book constitutes the proceedings of the 4th 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.