MapMark4 Shiny is a self-contained R Shiny app designed for quantitative mineral resource assessments. The target audience consists of practitioners of the three-part mineral resource assessment methodology (Singer and Menzie, 2010). The app performs calculations needed to combine information from grade-tonnage models with estimates of undiscovered deposits within permissive tracts. The MapMark4 Shiny app is an implementation of the MapMark4 R package (Ellefsen, 2017) that relies on R Portable version 3.6.1 (Huber, 2019; available at https://sourceforge.net/projects/rportable/). Consequently, no additional software installation is required on the user's behalf. The design of this app borrowed generously from Pang (2014). MapMark4 Shiny serves as an alternative, simplified version of the MapMark4 GUI (Shapiro, 2018) downloadable at https://pubs.er.usgs.gov/publication/tm7C18. Advanced users are encouraged to use the previously published MapMark4 GUI if control over specific parameters is required.