The fourth edition of the SEST conference series will be held in Vaasa, Finland, from 6-8 September 2021. Smart energy systems present enormous engineering challenges in the design and integration of energy and electrical grids with communication and network technologies, along with substantial questions of security and privacy of different components within the grid. The SEST conference aims at providing an opportunity to discuss various engineering challenges of smart energy system design and operation by focusing on advanced methods and practices for designing different components and their integration within the grid. It also provides a forum for researchers from academia and professionals from industry, as well as government regulators to tackle these challenges, and discuss and exchange knowledge and best practices about design and implementation of smart energy systems.