Karabiner (https://pqrs.org/osx/karabiner/index.html.en) Configuration for remapping "Sculpt Ergonomic Desktop" mouse's Windows button to Mouse Button 4. Derived from http://blog.yorkxin.org/posts/2014/04/12/microsoft-sculpt-mobile-mouse-and-mac/.

private.xml

<?xml version="1.0"?>
<root>
  <devicevendordef>
    <vendorname>MICROSOFT</vendorname>
    <vendorid>0x045e</vendorid>
  </devicevendordef>

  <deviceproductdef>
    <productname>SCULPT_ERGONOMIC_DESKTOP</productname>
    <productid>0x07a5</productid>
  </deviceproductdef>

  <item>
    <name>Windows Button to Mouse Button 4 (Microsoft Sculpt Ergonomic Desktop Mouse)</name>
    <identifier>com.microsoft.mouse.sculpt_ergonomic_desktop.win_button</identifier>
    <device_only>DeviceVendor::MICROSOFT,DeviceProduct::SCULPT_ERGONOMIC_DESKTOP</device_only>
    <autogen>
      __KeyToKey__
      KeyCode::OPTION_R,
      KeyCode::VK_MOUSEKEY_BUTTON_BUTTON4
    </autogen>
  </item>
</root>