Currently, mouse configuration is not a trivial task. Many new devices come  
with more than 3 buttons in addition to the mouse wheel. Buttons 4-7 are in  
practice reserved for the mouse wheel, which means extra buttons should begin  
counting from 8, but they do not.  
  
When mapping the wheel to buttons 4 and 5, extra buttons are by default  
ignored. Patches are available to "push" the button assignment when the wheel  
is detected, but this often means extra buttons get assigned as button 6 and  
up. This means that extra buttons will be treated as a horizontal scroll  
wheel, which is not the desired behavior.  
  
To make a 5 button mouse with wheel work "properly", you have to first  
configure the mouse with 7 buttons, with the wheel mapped to buttons 6 and 7:  
  
        Option "Buttons" "7" 
        Option "ZAxisMapping" "6 7" 
  
Then, once the X server is running, you have to use xmodmap to remap the  
buttons: 
 
        xmodmap -e "pointer = 1 2 3 4 5 8 9" 
 
Documentation explaining the proper setup would certainly help, along with an 
explanation as to why extra buttons begin at 8 instead of 4. This would also 
server as an official statement from X.org to toolkit and applicaiton 
developers as to how the mouse wheel is presented in the X11 protocol. Support 
in the X server for mapping extra buttons starting at 8 would be even better. 
The ideal solution is a protocol revision or extension that added a wheel 
event type (and enumerated buttons in logical order), but this requires 
changes to existing applications and toolkits which won't happen quickly.