The rotor hub flow prediction workshops are held bi-annually at Penn State's Vertical Lift Research Center of Excellence (VLRCOE) to support academia, industry, and government in validating state-of-the-art computational fluid dynamics (CFD) tools. Initiated with support from VLRCOE, the National Rotorcraft Technology Center (NRTC), and the Fluid Dynamics Research Consortium (FDRC) at Penn State, these workshops have been compelling examples of productive exchanges between experimentalists and computational researchers and have had positive impact on both planning new experiments and subsequent computational grid and method development. There is continued interest in these comparisons as rotor hub flows are characterized by complex turbulent flow fields, while at the same time being a primary contributor to helicopter parasite drag. High-Reynolds number testing of rotor hub flows is necessary as high-Reynolds turbulent coherent structures remain strong for long distances downstream of the hub and up to the long-age wake where they interact with the empennage and tail. Basic research conducted at Penn State's water tunnel facilities has provided such unique high Reynolds-scale data of rotor hub wakes, providing new data for physical understanding and CFD validation. This paper summarizes the experimental data campaigns and 'blind comparison' computational results presented at the fourth rotor hub flow prediction workshop.