Binding modifier key to mouse button 4 using KeyRemap4MacBook
I have no idea how much difference there is between KeyRemap4MacBook and Karabiner (minus Karabiner Elements), so I can't guarantee that this will work on older versions. KeyRemap4MacBook got a name change to "Karabiner" in 2014 I believe, so some of the documentation may not apply to it anymore.

I only have access to mac with Sierra, so I couldn't test it in any version.


Follow the instructions on how to add custom settings


Get back to this after you've completed step 3.

Ignore step 4 and replace the contents of private.xml with:




&lt;?xml version="1.0"?&gt;
&lt;root&gt;

  &lt;item&gt;
    &lt;name&gt;Mouse Button 4 to Shift&lt;/name&gt;
    &lt;identifier&gt;MouseBtn4ToShift&lt;/identifier&gt;
    &lt;autogen&gt;
      __KeyToKey__
        PointingButton::BUTTON4,
        KeyCode::SHIFT_L
    &lt;/autogen&gt;
  &lt;/item&gt;

&lt;/root&gt;





Continue the instructions on how to add custom settings at step 5.



  If you're not 100% sure that it's actually button number 4 that you're pressing, you can use the EventViewer to check.




Just incase someone is interested:


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