This book constitutes the refereed post-conference proceedings of the Fourth IFIP International Cross-Domain Conference on Internet of Things, IFIPIoT 2021, held virtually in November 2021.The 15 full papers presented were carefully reviewed and selected from 33 submissions.

Challenges in IoT Applications and Research.- Challenges in IoT Applications and Research.- Modernizing Agricultural Practice Using IoT.- sFarm: A Distributed Ledger based Remote Crop Monitoring System for Smart Farming.- Smart Agriculture using Flapping-Wing Micro Aerial Vehicles (FWMAVs).- Smart Lysimeter with Crop and Environment Monitoring: Enhanced with Pest and Crop Control.- Cyber-physical IoT Systems in Wildfire Context.- Fuel break Monitoring with Sentinel-2 Imagery and GEDI validation.- A Study on Small Sensor Node Antenna Performance Camouflaged under Grassland Fire.- A Data Fusion of IoT Sensor Networks for Decision Support in Forest Fire Suppression.- Drones as Sound Sensors for Energy-Based Acoustic Tracking on Wildfire Environments.- IoT for Smart Health.- cStick: A Calm Stick for Fall Prediction, Detection and Control in the IoMT Framework.- Hardware/Software Co-Design of a Low-Power IoT Fall Detection Device.- Security.- ReOPUF: Relaxation Oscillator Physical Unclonable Function for Reliable Key Generation in IoT Security.- Cyber-Physical Application for the Safety and Security Enforcement in Oil & Gas Transportation.- A Secure and Efficient Cloud-Connected Body Sensor Network Platform.- Methods.- EasyDeep: An IoT Friendly Robust Detection Method for GAN Generated Deepfake Images in Social Media.- A Fuzzy Logic Approach for Self-Managing Energy Efficiency in IoT Nodes.- Hydric Resources and Metereological Monitoring IoT System - The Software Architecture.