CMB-S4 will take the longstanding program of ground-based CMB experiments to unprecedented scope and scale in order to cross critical scientific thresholds over a broad range of fields.
Past generations of experiment have deployed a single size of telescope at a single site, observed at a small number of frequencies, and progressively increased their sensitivity by increasing their numbers of detectors from hundreds for Stage 1 experiments to thousands for Stage 2 and tens of thousands for Stage 3.
CMB-S4 will employ both large and small aperture telescopes in order to probe both small and large angular scales; it will deploy them at both the South Pole and in the Chilean Atacama Desert in order to be able to observed both small and large fields; it will use predominantly dichoric detectors (sensitive to two observing frequencies at once) to cover 11 observing bands in order to be able to remove contamination from galactic foregrounds such as dust and synchrotron; and it will deploy an order of magnitude more detectors than ever before in order to reach the sensitivity required for its science goals.
CMB-S4 will deploy the best proven technologies in the field, with the challenges primarily being in engineering them both to scale up by an order of magnitude and to achieve improved efficiency and uniformity. This applies both to hardware components such as the detectors and readout, and to the software required for data-acquisition and -management.
More details on the various experimental subsystems can be found on their individual pages.