MAX 4U: an upgrade of the MAX IV 3 GeV ring

The MAX IV 3 GeV storage ring in Lund, Sweden, is the first multibend achromat (MBA) lattice fourth-generation light source worldwide. In order to continue to offer the Swedish and international scientific communities state-of-the-art competitive tools beyond the end of this decade, MAX IV Laboratory launched in 2024 the conceptual design of MAX 4$^U$ , an upgrade of its 3 GeV storage ring aiming at an emittance below 100 pm.rad. This performance boost is to be achieved through a minimum-interference upgrade in which localized interventions in selected subsystems and components are carefully chosen to provide the maximum performance increase with minimum cost and, equally important, minimum dark time for the MAX IV user community. This contribution summarises the accelerator physics and engineering aspects of the MAX 4$^U$ conceptual design and presents the latest developments.