The changelog is tracked here but also in the Releases section of the GitHub project. The changelog only includes changes specific to the RubyGem.
The Bootstrap framework changes can be found in the Releases section of twbs/bootstrap. Release announcement posts on the official Bootstrap blog contain summaries of the most noteworthy changes made in each release of Bootstrap.
Fixes an extraneous sourceMappingURL in bootstrap.js.
#124
Compass is no longer supported. Minimum required Sass version is now v3.5.2. #122
This release corresponds to the upstream Bootstrap 4 Alpha 3.