The Fourth Workshop on Hot Topics in Software Upgrades (HotSWUp 2012) was held on June 3, 2012 in Zurich, Switzerland. The workshop was co-located with ICSE 2012. The goal of HotSWUp is to identify, through interdisciplinary collaboration, cutting-edge research ideas for implementing software upgrades. The workshop combined presentations of peer-reviewed research papers with a keynote speech on how empirical software engineering can help reduce update-induced failures. The audience included researchers and practitioners from academia and industry. In addition to the technical presentations, the program allowed ample time for discussions, which were driven by debate questions provided in advance by the presenters. HotSWUp provides a premier forum for discussing problems that are often considered niche topics in the established research communities. For example, the technical discussions at HotSWUp'12 covered dynamic software updates, package management tools, using model-checking and verification to verify updates, empirical software engineering and repository mining, and highlighted many synergies among these and other topics.