Mapping a keyboard key to mouse button 4 or 5.

Hello Xperts,

I have a 3-button trackball and like the EmulateWheel
option, but if I do

  Option   "EmulateWheel"   "true"
  Option   "EmulateWheelButton" "2"

then the middle button can't be used for any purpose
other than toggling the wheel emulation when it's
held.

If I hold shift and press num lock, then I can use the
numeric keypad to generate mouse clicks, but you can
only generate button 1, button 2, and button 3 events.

I would like to be able to put something in my
.Xmodmap (or whatever) so that there is keyboard key
that emulates mouse button 4. That way, I can put

  Option   "EmulateWheelButton" "4"

into my /etc/X11/XF86Config-4 so that when I press and
hold my keyboard key that does button 4, I can move
the trackball wheel up and down to do scrolling in
applications like Mozilla.

Thanks in advance for any advice.

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