In the coming decade, we foresee a rich and fruitful evolution in the field of cosmic microwave background (CMB) observations. Polarization measurements at degree angular scales constrain possible primordial gravitational waves, probing inflation and quantum fluctuations of gravity. Polarization fluctuations at arcmin angular scales, on the other hand, trace the growth of large-scale structure via gravitational lensing effects. These data sets are rich for measuring dark energy, the neutrino mass, and the evolution of galaxies and clusters. The Stage-4 CMB experiment (CMB S4) is a program to measure the CMB polarization with a sensitivity orders of magnitude better than that of current experiments. In this talk, I will review some challenges we are facing with this project and discuss how we aim to overcome these challenges, using examples of past and current experiments such as the Atacama B-mode search and the Simons Array. I will also discuss our R&D work to develop a cryogenic half-wave plate (HWP) that will function as a polarization modulator for CMB S4. The goal of this HWP is to achieve necessary systematic error reduction.