The document outlines guidelines for writing a 4-star impact case study, emphasizing the need for underpinning research of at least 2-star quality published after 2000 and impacts demonstrated between August 2013 and July 2020. It stresses the importance of evidence-based testimonials and clear connections between research and impacts, highlighting differences between high and low-scoring case studies. Moreover, it provides insights into qualitative and quantitative aspects that contribute to obtaining a high score in evaluations, particularly in the context of REF2021.