If you're already running a previous beta of Firefox 4 you should see an automatic update soon. Alternatively, you can download Beta 11 directly for Windows, Mac OS X, and Linux 32-bit/64-bit. The beta supports 76 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. There's still a Beta 12 in the works before we see the release candidate and final builds arrive.
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.