While we wait for the official release notes for Firefox 4 RC, you may hit the following direct links to download the Firefox 4 Release Candidate 1 installer for your computer (Windows / Mac / Linux).
Firefox 4 RC Direct Download Links:
Windows [12 MB]
Mac OS X [26.8 MB]
Linux [13.2 MB]
You can also try this tip to install both Firefox 3 and Firefox 4 on the same computer.