You can download Firefox 4 from [http://releases.mozilla.org/pub/mozilla.org/firefox/releases/4.0.1/win32/en-US/ this link]. However, please be aware that Firefox 4 contains security bugs that are fixed in the [http://getfirefox.com/ latest version, Firefox 5]. If you have add-ons that are marked compatible with Firefox 4 but not Firefox 5, you can use the [https://addons.mozilla.org/firefox/addon/add-on-compatibility-reporter/ Add-on Compatibility Reporter] to enable them in Firefox 5 and report any problems that occur.