Use the following options in your XF86Config-4
Option "Protocol" "ExplorerPS/2"
Option "Emulate3Buttons" "false"
Option "ZAxisMapping" "5 6"
Option "Buttons" "6"

Then use:xmodmap -e "pointer = 1 2 3 6 4 5"
to get button 6 to generate a unique event.

Instructions work fine, but mapping reset everytime, modify /etc/X11/Xmodmap control file and add line

pointer = 1 2 3 6 4 5

this will no set it up correctly everytime xwindows loads