A new stable release of MPICH, 4.0, is now available for download. This is the first stable release in the 4.0.x series. In addition to support for all new MPI-4.0 APIs, this release contains significant advances in GPU and multithreaded application support. Also included are numerous bug fixes and code quality improvements.
See https://github.com/pmodels/mpich/blob/v4.0/CHANGES for a summary of all changes since v3.4.3. The full list of commits can be found here.
Production environments are encouraged to upgrade to this release.