Try XButton1 and XButton2

quote from the autohotkey helpfile:

The fourth button (XButton1) and the fifth button (XButton2) might be assignable if your mouse driver allows their clicks to be seen by the system. If they cannot be seen -- or if your mouse has more than five buttons that you want to use -- you can try configuring the software that came with the mouse (sometimes accessible in the Control Panel or Start Menu) to send a keystroke whenever you press one of these buttons.
If it doesn't work use your mouse software to assign button 4 and button 5 to an obscure keypress.