Four recommendations for open source software (4OSS lesson) ----------------------------------------------------------- **Dimitrios Bampalikis, Leyla Jael Castro, José María Fernández González, Eva Martin del Pico, Fotis E. Psomopoulos, Allegra Via** Scientific data-driven research relies on software to create, process and analyze data, yet software is not always developed following practices that ensure its quality, sustainability and FAIRness. Rather than proposing new software development best practices, this course aims to provide researchers using some code in their projects with four simple recommendations encouraging the adoption of existing best practices. Software development best practices promote better quality software, and better quality software improves the reproducibility and reusability of research. These four recommendations are designed around Open Source values, and provide practical suggestions that contribute to making research software and its source code more discoverable, reusable and transparent. This course is aimed at researchers working with software (or willing to do so) as part of their regular activities.