Username or EmailPassword
it is already avaliable for linux
you have to extract the folder, put it where you want (eg: /usr/local/firefox4) and edit your typical shortcut to point to the executable file (eg: /usr/local/firefox4/firefox). I much more prefer to install things through Ubuntu Software Center or apt-get, but for Firefox 4 betas I am doing an exception.
If you just want to test the beta without affecting your normal FF profile, you can install the FoxTester extension. Point that at a directory, then copy the .tar.bz2 file for whatever version you want to test into that directory, and then you can access them via the FoxTester menu.
Run multiple versions at once, without installing any of them.