CLUSTER4 is dedicated to the memory of our friend Mario Nonino (Udine 26/11/1959 – 21/07/2023). Mario's career in Astronomy started with his doctoral work on Seyfert galaxies under Margherita Hack in 1989. Over the years, his broad scientific interests ranged from the formation and evolution of galaxies in clusters and the field, to the search for distant galaxies and the study of stellar populations in our Galaxy and in the nearby Universe. His unique international experience in planning, executing, and leading the scientific exploitation of data from cutting-edge telescopes, both ground-based and space-borne, earned him prominent roles in major cosmological surveys. Mario's latest achievement as principal investigator was the ESO Public Survey GCAV (Galaxy Clusters At Vircam) using the VISTA telescope. His discovery of a brown dwarf from JWST data is a recent example among his numerous contributions to the field, consolidating his role in large international collaborations. As the ESA Euclid satellite's launch approached, Mario took a coordinating role within Ou-Mer, highlighting his passion for research driven by curiosity, technical challenge, and the joy of unexpected discoveries. His wit, irony, and immense scientific and professional value, paired with modesty, generosity, and humanity, made working alongside him a pleasure and privilege. Mario's legacy as an invaluable mentor and a problem-solver who viewed challenges as opportunities will forever inspire his friends and collaborators.