For some odd reason, if you have a 5-button mouse, the 5th button (aka “forward button”) in emacs has syntax <mouse-5> in Windows and Mac, but on Ubuntu 10.04 Linux running Gnome 2.x, it's <mouse-9>.
In unix X11, here seems to be how they assign the mouse buttons.
In emacs, you can find out the mouse button or wheel syntax by calling “describe-key” then press the button.
In X11, you can call “xev” to see any keyboard/mouse event and their naming.
The numbering also depends on OS and mouse driver.
For example, on Mac OS X 10.4.x, using a Microsoft 5-buttons mouse, the numbering changes depending on whether you have Microsoft's mouse driver (IntelliPoint) installed.
If you don't have IntelliPoint installed, then Mac OS X doesn't register the mouse wheel tilt left/right, and your Backward (4th) button is the same as right-button, and Forward (5th) button is 4th button.
Here's the numbering reported by emacs's “describe-key”.
<mouse-1> = left button<mouse-2> = middle button (pressing the scroll wheel)<mouse-3> = right button<wheel-up> = roll scroll wheel up<wheel-down> = roll scroll wheel down<mouse-3> = 4th button (aka backward button)<mouse-4> = 5th button (aka forward button)