When using the Kensington Expert Mouse as is, the 4 buttons are physical buttons to LEFT, RIGHT, MIDDLE and BACK. To get both BACK and FORWARD a simple rebind is possible by adding an appropriate section to the xorg.conf (BACK -> FORWARD and MIDDLE -> BACK)
┌─┬─┐ ┌─┬─┐ │M│B│ │B│F│ ├─O─┤ -> ├─O─┤ │L│R│ │L│R│ └─┴─┘ └─┴─┘
If I now additionally want a MIDDLE button the man pages mention the option "Emulate3Buttons", which emulates clicking MIDDLE by pressing both LEFT and RIGHT together.
My problem now is this: "Emulate3Buttons" emulates a physical MIDDLE button and since I had to map MIDDLE -> BACK earlier, I will just get another BACK button.
┌─┬─┐ │B│F│ ├─O─┤ │L│R│ └\┴/┘ B
How would I go about to end up with:
┌─┬─┐ │B│F│ ├─O─┤ │L│R│ └\┴/┘ M