I recently upgraded from debian's exim 3.x to gentoo's 4.x. As a part of the install process, a perl script is generated that will convert your version 3 configuration file over to a version 4 config file. Gentoo deletes this conversion script by default. In the future, it might be an idea to drop that script somewhere, maybe in /tmp so it can be used as intended.
Both the convert4r4 and convert4r3 files are now installed by the exim-4.05 ebuild. Thanks for the suggestion!