If you're already running a previous build of Firefox 4 you should see an automatic update soon. Alternatively, you can download RC1 directly for Windows, Mac OS X, and Linux 32-bit/64-bit. The RC supports 79 languages.
Mozilla was originally planning on having Firefox 4 out by the end of last year, but it has had to delay the release multiple times. The company has fixed more than 7,000 bugs in the 12 betas. After this RC build there will likely be at least one more, followed by a final release.
Mozilla has been struggling to get Firefox 4.0 out the door. As a result, the company has announced that in 2011, not only will we see Firefox 4.0, but we will also see Firefox 5.0, Firefox 6.0, and Firefox 7.0.