This book constitutes the refereed proceedings of the 4th International Symposium on Human Mental Workload: Models and Applications, H-WORKLOAD 2020, held in Granda, Spain*, in December 2020. The volume presents one keynote paper as well as 13 revised full papers, which were carefully reviewed and selected from 22 submissions. The papers are organized in two topical sections on models and applications. *The conference was held virtually due to the COVID-19 pandemic.