DocBook V4.1 and DocBook XML V4.1.2 became an OASIS Standard in February 2001.
DocBook V4 introduced a number of backward-incompatible changes.
The policy of the DocBook TC is that backward-incompatible changes can only be introduced in a full version release. They must also be described in comments in the DTD at least one full version earlier. In other words, all the planned version 4.0 incompatibilities were announced with version 3.0.
Backward-incompatible changes to DocBook that are planned for version 4.0:
The DocInfo element will be split out into ChapterInfo, AppendixInfo, and so on. ArtHeader will be renamed to ArticleInfo. SeriesInfo may be discarded because it has become a special case of BiblioSet. BookBiblio will be discarded in favor of a new, more inclusive, structure for BookInfo (and for ArticleInfo, whose earlier ArtHeader form contained BookBiblio).
The article.class entity may be removed from the Book content model, and made part of a new top-level document hierarchy.
The nav.class entity, which appears in several divisions, will allow ToCchap instead of ToC.
MsgText will be moved from tech.char.class to a more appropriate parameter entity.
The following elements will have their content constrained to the smallcptr.char.mix mixture: Action, Command, Database, Filename, Hardware, Interface, KeyCap, Literal, Option, Parameter, Property, and SystemItem.
AuthorBlurb and Affiliation will be removed from person.ident.mix and a new wrapper element will be created to allow association of those two elements with Author name information.
Epigraph will not be appearing in BlockQuote.
Comment will be renamed to Remark and will be excluded from itself.
GlossTerm will be excluded from itself, and may be split into an element that appears in a Glossary and an element that can appear in the main text.
The Subject attribute on GlossDef will be renamed Keyword.
Two SegTitle elements will be required in SegmentedList.
Graphic and InlineGraphic will be declared EMPTY. This change will require that end tags be removed and that any embedded graphic content be stored outside the SGML source and pointed to from an Entityref or Fileref attribute.
The OptMult and ReqMult values for the Choice attribute on Group will be removed. Use the Rep attribute instead to indicate that the choice is repeatable.
The content model group inside FuncSynopsis starting with FuncDef will not be available; you will have to use FuncPrototype. Also, you will be able to have a mixture of FuncPrototypes and FuncSynopsisInfos (this is not backward-incompatible all by itself).
The EnvironVar and Prompt values for the Class attribute on SystemItem will be eliminated; use the EnVar and Prompt elements instead.