This book collects the main contributions from the 4th edition of the NICFD conference, organized by the Special Interest Group on Non-Ideal Compressible Fluid Dynamics (SIG- 49). It provides some of the latest research findings in the field of NICFD, relevant to a number of engineering applications related to the conversion of renewable and waste energy sources, like, e.g., organic Rankine cycles, super-critical CO₂ cycle power plants, combustors operating with supercritical fluids, and heat pumps. The book reports on research encompassing theoretical, computational, and experimental aspects of the gas-dynamics of non-ideal reactive and non-reactive flows and their impact for the design of internal-flow components (turbomachinery, heat exchangers, combustors). All chapters address challenges related to characterizing the behaviour of non-ideal fluids, where state-of-the-art models are used to predict the thermo-physical properties of both pure and multi-phase fluids.