Interferometer Configuration and Commissioning of the Virgo Detector during the O4b/c Run

Abstract On 10 April 2024 the Virgo interferometer joined the international gravitational-wave network for the second and third parts of the fourth observing run, O4b and O4c, the latter of which will end on 18 November 2025. The Virgo detector is the first large-scale detector running with a marginally stable Signal Recycling Cavity, a configuration which poses significant challenges from the optical, thermal and control points of view. In this article a description of this configuration of the Virgo detector will be given. As a follow-up to the configuration in use at the beginning of the Run, an extensive report of the main commissioning activities performed during the run will be given. The goal for these activities is two-fold: one important aspect is maintaining, if not improving, the performance of the detector over time, profiting of the longer time scales during the observing run to better understand the behaviour of the detector. The second important aspect is to characterize better the noise content of the detector, and study in advance its characteristics in view of the future developments which are already planned for the next observing run, O5. Both aspects will therefore shape the Virgo detector for the last part of the observing run, and will link to its evolution for the next years.