Changes since version 4.3 include new
biblioref elements and a fix for a bug in the catalog files distributed in the 4.3 release.
You can download the 4.4 beta release in a variety of formats:
Get the RELAX NG version and give it try with James Clark’s nXML mode for GNU Emacs.
Version 4.4 may well be the final 4.x release. The next left-of-dot version, DocBook 5, will be RELAX NG-based and will probably be a sort of evolutionary refactoring of DocBook. For Norm Walsh’s thoughts about just what kind of refactoring it might be, see the DocBook Evolution thread at Norm’s weblog.
Norm has been developing a prototype “next generation” version of DocBook, DocBook NG. The latest version of that is the Cachaça release (but you can still get the previous Bourbon and Absinthe releases if you want).