<EDIT>Revised to fix which fingers Neo-Geo Games are supposed to use</EDIT>
>What games do you actually use that thumb button for?
Rarely used.
>I understand it would partially be Neo-Geo games, but isn't that 4th button almost optional in >those few games? Or not a critcal function?
Depends - First off, the classic Neo-Geo was set up to be
2 3 4
1
With 1 being primary and pressed with the thumb. That thumb button becomes important for that.
(and a lot of people, including me, used to think the button layout was Index to pinky).
I think some people may be playing the games -
1 2 3
4
With the thumb button activating button 4. It is less critical there.
I know the baseball games use Button 4 for Time out. It is also used in NG fighters, but I don't play those.
>And, other that Neo-Geo, what games do you use that button for?
I would think Stargate - maybe Space Duel.