The latest release is 4.20. This is a minor upgrade on the 4.1x series of releases, however the documentation has been updated to match.
New features are documented in the CHANGES and NewStuff files.
All versions of exim previous to version 4.x are now considered obsolete and everyone is strongly recommended to upgrade to a current release.
The last 3.x release wass 3.36.
There may be beta versions with available from the ftp sites in the Testing directory. Many people are using these without problems, but they are not recommended unless you are willing to work with beta software.