v4 is semi-retired, which means that we will provide very limited support to v4 users who have not yet upgraded, and will provide assistance in performing upgrades. However, we will no longer ship maintenance releases, patches, or other changes.
The central theme of v4 was all about universal package management, which meant managing all of the various types of packages and feeds in the enterprise.
v3 has been retired and is no longer supported.
The central theme of v3 was to extend the reach of package serving to other registries/repositories including npm.
v2 has been retired and is no longer supported.
The central theme of v2 was to introduced more filtering options (particularly for license types), and added meta-features such as the SDK, the API, and source code licensing.