For Defender's reverse button I am wagering...
Nah - I built a repro defender control panel for that.

The game really isn't much fun unless you have the orginal setup.
The 4th button is just for games that needed 4 buttons. I was used to the thumb position from playing a lot of Mortal Kombat, and it saved space.
To be honest, I haven't needed the 4th button much at all -- I would recommend people either go with 3, or if you like the fighting games, go with 6-7. I ended up building a Mortal Kombat 4 repro panel.