The NET4EXA project aims to create an advanced interconnect for HPC and AI systems, including Large Language Models, that will scale to hundreds of thousands of computing nodes. The interconnect forms the backbone of such systems, and their overall performance is intimately tied to the scalability of the network and its tight coupling with the high-end compute elements. Building on the successful BXI* European HPC Interconnect, whose earlier versions (BXIv1 and BXIv2) have been deployed in European supercomputers ranking among the TOP15, and based on the research for its evolution carried out in the RED-SEA and other previous European RIA projects, NET4EXA will pursue the development of the next generation of European Interconnects (BXIv3). This includes both the switch and network interface, encompassing both hardware and its associated software. The project will showcase a fully functional Pilot at TRL 8, poised for adoption and integration into exascale and post-exascale European supercomputers. NET4EXA will also prepare the subsequent version, BXIv4, planned for implementation after this Innovation Action. The project will contribute by offering analysis and preliminary design for upcoming interconnect features, thus paving the way for continued progress in the field. Leveraging two decades of BULL experience in building HPC solutions, with now more than 40 Top500 entries in 2024, the project outcomes will fit the technological requirements for state-of-the-art HPC solutions to be implemented in European and international supercomputers and datacentres from 2025 onward. *BXI: BullSequana eXascale Interconnect