Besides fixing plenty of little bugs, the major enhancements most notable are probably the fixes to the feeds, the greater flexibility for comments and previews (new tags and new attributes) and the custom error-pages.

Here is the changelog (changes since 4.0.1):

• Fixed a bug in 404 handling on some fastcgi servers
• Fixed a bug that prevented file_download_list from working properly
• Fixed a redirection bug in the comment form
• Fixed occasional category URL i18n problems
• Removed extra trailing slash in some breadcrumb links
• Made articles by author work properly
• Made feed_link support section and category correctly
• Added correct content type to popup comments page
• Fixed an obscure feed caching bug
• Fixed a possible circular dependencies problem when loading plugins
• Fixed timestamps in Atom feeds
• Fixed occasional Atom encoding issues
• UI improvements when creating/copying/deleting styles
• Improved performance when loading languages
• Made date-based permlinks work properly in comment emails
• Better handling of invalid login cookies
• Added page templates for 404 (and other) error pages
• Minor Atom validation improvements
• Improved comment conditionals logic – <txp:if_comments> now means ‘if the current article has at least 1 comment’
• Minor performance improvements
• New <txp:if_status> and <txp:page_url> tags
• config.php whitespace no longer breaks stylesheets
• UI improvements to javascript toggles
• Improved comment input form HTML markup
• Improved txp:section and txp:category tags
• New tags: <txp:if_different>, <txp:if_first_article>, <txp:if_last_article>
• Localized the XML feed link title
• Better encoding of article titles
• Fixed rare problems caused by magic_quotes_runtime
• Improved file upload forms
• New language: Latvian
• Added support for section and category titles in txp:breadcrumb
• decimals="0" in <txp:file_download_size> tag now allowed
• Minor diagnostic info changes
• Added new labeltag attribute on most list tags
• Restored use_textile preference
• Minor HTML validation fixes
• Developers: added new safe_upsert() database function
• Now falls back to English if no language data is available
• Support comma-separated names in <txp:if_category> and <txp:if_section>
• Offending tags are now named when displaying error messages in templates and forms
• Added include_default attribute to <txp:section_list>
• Added support for changing status and section on multiple articles at once
• Fixed nested category titles in txp:breadcrumb
• Improved comment message markup
• Restored the “comments closed” message
• Better handling of per-article ‘Use Textile’ options
• Added CSS class attribute to <txp:comments_invite>
• Added 4 attributes to <txp:comments_invite>: showcount, wraptag, textonly (just the invite) and showalways (also display on individual article pages). Example: <txp:comments_invite wraptag="p" showcount="0" textonly="1" showalways="1" />
• C-style backslash sequences in tag attributes are now supported
• Fixed some rare bugs in feeds, admin list paging
• Added a pageby attribute to <txp:article>, for multi-column lists
• Fixed link_to_next/prev so they work at the top of a page
• Minor speed improvements
• New class and linkclass attributes for <txp:breadcrumb> tag
• Added <txp:comments_preview> and <txp:if_comments_preview> tags
• Fixed unparsed excerpts in feeds
• Textile: smarter handling of relative link and image paths
• Textile: stopped footnote links from clashing in article lists
• Add fi-FI locale support

Comments are welcomed in the forum thread.