Software is fundamental to research, fulfilling many different roles - instrument, model, tool, infrastructure - across all disciplines. Recent shifts in the wider research landscape, e.g. inclusion of research software in policies developed by the OECD and UNESCO, necessitate new approaches to software sustainability and consolidation and scaling of existing initiatives to support research software (the software used in research) and digital research infrastructure (the compute, data, networking, software and people infrastructure) that enables it. Thus far, support for research software has tended to focus on individuals or national policies and standards. Moving forward, organisations such as universities and other research institutions will play an increasingly important role in ensuring research software culture and practice is adopted by the research community. This is essential to empower those engaged in research to fully harness the potential of software and foster the execution of excellent research. The Software Sustainability Institute (SSI) was established in 2010 as the first organisation in the world dedicated to the development and support of research software best practices. In its first phase (2010 - 2015), the SSI gained an understanding of the state of the nation of research software, its developers/users, its requirements, and the importance of software for conducting research. The second phase (2015 - 2019) focussed on supporting communities to become self-sustaining and campaigning for change in research culture. In the third phase (2018 - 2024), the SSI consolidated its position as world-leading experts in research software policy and best practices. The SSI also scaled up its highly successful activities to make them more sustainable in the longer term. Throughout, the SSI has fostered a large, collaborative, worldwide community of advocates and practitioners to help deliver on their motto: better software, better research. The fourth phase of the SSI will continue enhancing and scaling its signature activities, including the fellowship programme, community building, career development and training. It will continue to campaign for the recognition of all of the people and outputs that contribute to research and add a new focus on environmental sustainability and empowering organisations to take responsibility for the research software they create and use. Four impacts will guide the work in SSI-4: 1. Evidence-driven research software policy and guidance. 2. Capable research communities. 3. Widespread adoption of research software best practice. 4. Broadened access and contributions to the research software community. The SSI will achieve these through: - Building on its successful platforms and campaigns: empowering individuals through the Fellowship Programme, amplifying dissemination through online resources and social media, raising awareness of research software through events, and campaigning for policy and research culture change. - Growing its policy and research activities: building on SSI national landscape studies, collaborating on the HiddenREF campaign, creating new connections to further embed software into UK research policy. - Developing new training courses, learning pathways, communities of practice handbooks, and bringing the community together through the Collaborations Workshop. - Co-producing research to explore the barriers and enablers to career progress, commissioning articles and guides from a diverse range of authors, and running workshops in other, non-English, languages. - Coordinating an innovative software funding pilot to better understand how research software maintenance and development should be funded.