• Firefox 4 Beta 4 is available in 39 languages.
• Firefox Sync is now included by default.
• A new feature gives users a visual overview of all open tabs, allowing them to be sorted and grouped.
• An experimental API is included to provide more efficient Javascript animations.
• Firefox now supports the HTML5 video "buffered" property.
• See the complete changelist from the previous beta.

As well as these features from previous Firefox 4 Betas:

• Tabs are now on top by default on Windows and OSX - Linux will be changing when the theme has been modified to support the change.
• You can turn any tab into an "App Tab" by right-clicking on it and selecting "Make into App Tab" from the context menu.
• Web developers can animate content using CSS Transitions.
• Responsiveness and scrolling improvements from the new retained layers layout system.
• JavaScript speed improvements due to engine optimizations.
• Changes to how XPCOM components are registered in order to help startup time and process separation.
• You can search for and switch to already open tabs in the Smart Location Bar.
• New Addons Manager and extension management API (UI will be changed before final release).
• Significant API improvements are available for JS-ctypes, a foreign function interface for extensions.
• The stop and reload buttons have been merged into a single button on Windows, Mac and Linux.
• The Bookmarks Toolbar has been replaced with a Bookmarks Button by default (you can switch it back if you'd like).
• Crash protection for Windows, Linux, and Mac when there is a crash in the Adobe Flash, Apple Quicktime or Microsoft Silverlight plugins.
• CSS Transitions are partially supported.
• Full WebGL support is included but disabled by default at this time.
• Core Animation rendering model for plugins on Mac OS X. Plugins which also support this rendering model can now draw faster and more efficiently.
• Native support for the HD HTML5 WebM video format.
• An experimental Direct2D rendering backend is available on Windows, turned off by default.
• Web developers can use Websockets for a low complexity, low latency, bidirectional communications API.
• Web developers can update the URL field without reloading the page using HTML History APIs.
• More responsive page rendering using lazy frame construction.
• Link history lookup is done asynchronously to provide better responsiveness during pageload.
• CSS :visited selectors have been changed to block websites from being able to check a user's browsing history.
• New HTML5 parser.
• Support for more HTML5 form controls.
• Web authors can now get touch events from Firefox users on Windows 7 machines.
• A new way of representing values in JavaScript that allows Firefox to execute heavy, numeric code (used for things like graphics and animations) more efficiently.