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CMB Stage-4 (CMB-S4) is a coordinated effort to develop new ground-based cosmic microwave background (CMB) observations, begun around 2015 and aiming at deployment roughly in the 2020s. As of 2020, the effort still consists of proposals regarding the means by which the vision might be achieved. The observations would be follow-ups to the mid-2010s experiments such as Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT). The initiative aims to deploy new technology at both locations (the South Pole and the Atacama Desert) as well as consideration of adding a site (or sites) more northward. The vision is telescopes and associated surveys with receiver arrays that have an order-of-magnitude more receiver-elements (e.g., bolometers) than current equipment, i.e., an array would have on the order of 100,000 elements.