Steve just posted on his blog the official announcement: Subtext v1.9.4 "Windward" Edition Released
My blog is running it since a few weeks with no issues, so, I think it's safe to upgrade it.
As I previously said, the most important feature, IMHO, is the Sitemap implementation made by David Vidmar, from Slovenia. For more information about that feature, and for instructions on how to set it up, I recommend his post: Subtext 1.9.4 - now with sitemap.
The final release have just one small difference: the sitemap is inside a "sitemap" folder instead of the root folder of the blog, so the final url will be http://blogUrl/sitemap/sitemap.ashx.
For all the other new feature are the release notes, and you can download the official 1.9.4 version (release 78) from Sourceforge.