Mozilla has officially announced Beta 4 of Firefox 4.0 and its available for download from Mozilla website.
It has Following important changes:
* Firefox 4 Beta 4 is available in 39 languages.
* Firefox Sync is now included by default.
* Users can organize their tabs using Firefox Overview.
* Firefox now supports the HTML5 video “buffered” property.
And also latest Firefox 4 Beta 5 Preview build is available for users. If you want to test it the get it from following direct link.