And what tells the section:

Code:
Section "InputDevice"
        Identifier	"USB Mouse"
        Driver	"mouse"
        Option	"Device"	"/dev/input/mice"
        Option	"SendCoreEvents"	"true"
        Option	"Protocol"	"IMPS/2"
        Option	"ZAxisMapping"	"4 5"
        Option	"Buttons"	"5"
EndSection
in /etc/X11/XF86Config-4?

Of course you look only for 'Driver "mouse"' - not USB Mouse etc.