Current dev status of the standard edition of the software itself is 95% complete. The missing bits are:
* Read-receipts
* FileEmbedder plugin
* Support for a few charsets (Shift-JIS, Big5)
* Version 3 compatibility wrappers
There's another edition of version 4 (enterprise) which will be available however. A more feature-rich business-oriented version which comes under a non-free license. That isn't so complete right now and until it's finished there wont be a non-beta standard edition out. The enterprise edition effectively builds on top of the standard edition so there's potential for significant changes even at this stage.
Because of the business side of this new version the website needs a complete rewrite. Most of the current site is static content. The new website needs a whole e-commerce module writing and integrating, along with a bug-tracker (can't use SF.net for licensing reasons) for the enterprise version (priority bugfixes) and a licensee download area. Actual written documentation is missing too. I'm also releasing an e-book.
Basically I have my work cut out.
I'm waiting on a designer to finish the website template re-design.
All that said, because the standard edition (the same featureset which v3 has, plus a bit more) is nearly done I'll be polling for beta testers soon. The beta will be run in two stages:
1) Roughly 100 volunteer beta testers will be provided with the beta software and a bug tracker to feedback to me
2) Once the private beta is over and any changes have been made, an official public beta will be released and made available for download on the website. Even if this is considered stable it will remain beta until all the stuff I mentioned above is ready.
I really wish I could do this full-time, but unfortunately I have to work 8 hours a day and cycle 1 hour each direction, eat my dinner and of course sleep! That leaves me a short amount of time to do any work on it, but I do chip away day by day
Of course, there's nothing stopping the curious from checking out from subversion. The stuff which is already there does work, you just wouldn't under any circumstances want to use it in production. It's also undocumented other than comments in the code, and the test coverage.
https://swiftmailer.svn.sourceforge.net ... simplified
EDIT | If ZenCart supports PHP4 they won't be able to use Swift Mailer v4 anyway

It's a PHP5-only release.