Although Mozilla Firefox 4 plan for release on March 22,2011,but now you can download Mozilla Firefox 4 (English version) from Mozilla FTP server and have first trial run.
Go to ftp://ftp.mozilla.org/pub/firefox/releases/4.0/
,click ‘Win32’ folder if PC OS is Windows version and select ‘en-US’ folder,
You may notice Firefox Setup 4.0 exe file already appear inside.Download it .
Mac and Linux Mozilla Firefox 4 version please check the FTP server folder.