CMB-S4 will build on the work done by the current generation of CMB telescopes, including SPIDER, to obtain even more precise measurements of CMB polarization across a wide range of angular scales. This resulting dataset will probe a variety of different topics including inflation, neutrino physics, and dark energy. While this project is still in its early planning stages, it has the potential to answer many outstanding questions in cosmology and represents a major technological milestone for millimeter-wave instrumentation.