Bourbon 4 has a dependency of Sass 3.3+. Whereas Bourbon 3.2 has a dependency of Sass 3.2.

Bourbon v4 uses new Sass 3.3 features, like the string functions and maps seen in the completely rewritten @background and @background-image mixins: Issue #358. You can view more info about what's in Bourbon v4 versus v3.2 in the Backward incompatibility warning section on the releases page.

Bourbon v3.2 was originally not going to be released—the idea was straight to 4.0. But after discovering that Libsass (and sass-rails) are incompatible with Bourbon 4 because of the Sass 3.3 dep, I felt it would be alienating to those users to not provide a version of Bourbon 4.0 that is compatible with their setup. Therefore Bourbon 3.2 was released to continue to provide support for Libsass as much as possible. Libsass & sass-rails will hopefully support Sass 3.3 and thus Bourbon 4 in the future.

But more importantly Bourbon 4 paves the way for features and code that isn't hindered by the 3.2 dependency. If you can use Bourbon 4, I recommend you do so. But if you are held back by another dependency (libsass/sass-rails) then Bourbon 3.2 is for you.

The reason Bourbon released a 4.0 and not simply a 3.2 is becuase we introduced features that broke backwards compatibility. When breaking backwards compatibility it is recommended to update the Major version number. Bourbon follows the SemVer spec for semantic versioning.

I hope that sheds some light. Bourbon v4.0 and v3.2 are similar in all regards when it comes to the newly added features. Features that break backwards compatibility are only found in v4.

No worries, found the details here: RC1 and here: RC2