This editorial discusses the technological advancements in Gen4 nuclear energy systems and small reactors. These advanced systems, such as sodium-cooled fast reactors, lead-cooled fast reactors, and gas-cooled fast reactors, offer superior sustainability, safety, reliability, and economics compared to current-generation systems. The article highlights specific research on the safety analysis of a lead-based reactor, the operational characteristics of a micro gas-cooled reactor, the flow stability of supercritical carbon dioxide in a Brayton cycle, and the fault diagnosis of rotating machinery using deep learning models. The authors hope that these articles will serve as a valuable reference for researchers in the field of Gen4 nuclear energy systems and small reactors. [Extracted from the article]