This report is based on the discussions, presentations, and reflections on the Workshop on Sustainable Software Sustainability (wosss.org/wosss21/home) that took place in October 2021 (WoSSS21). The workshop brought together leaders, managers, and specialists in software sustainability. The topics covered included Open Science, FAIR software, human and technical infrastructure, advances in research software and software aspects of cultural heritage. The report captures the state of the art for a range of software sustainability topics according to those who attended. It contains featured sections (where authors provided information beyond a summary), summaries, discussions, and where applicable links to videos of the talks, slides and transcripts. This is the 4th report in the WoSSS report series (wosss.org).