MathJax v4 is the culmination of several years of work, and includes significant new features, as well as improvements on existing features. There are some potentially breaking changes, and in order to not have these changes affect existing web sites, this release increments the major version number so that sites using the mathjax@3 URLs will be protected from these until they update to version 4 explicitly. That is, this new release is an opt-in update.

See also the MathJax v4.0 release notes for a list of the commits and bug fixes in this release.