This book constitutes the refereed proceedings of the 4th International Workshop on Asynchronous Many-Task Systems and Applications, WAMTA 2026, held in Garching, Germany, during February 16–18, 2026.The 12 full papers and 2 Short papers included in this book were carefully reviewed and selected from 18 submissions. They focus on the advantages and challenges of task-based programming on modern and future HPC systems.