The Cosmic Microwave Background Stage Four Experiment (CMB-S4) is a planned DOE/NSF ground-based experiment, endorsed as a high priority mission in the Astro2020 report, with scientific impacts reaching from transformative measurements of the cosmic microwave background (CMB) to a deep legacy millimeter-wavelength dataset covering a large fraction of the sky. To meet its ambitious goals, CMB-S4 plans to have multiple small-aperture (0.55-meter) and large-aperture (6-meter) telescopes located both in the Chilean Atacama desert (to access a large fraction of the sky) and at the South Pole (for targeted deep-field observations). Over 500,000 superconducting detectors will be distributed across these telescopes, enabling a necessary leap in sensitivity. We present an overview of the project organization, technical design, construction plans, and predicted performance of CMB-S4. We highlight some driving programmatic and technical considerations of the current experimental design.
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