Output 4:3 video on old Apple TV?
I don't believe it will be 4:3. One of the requirements for ATV was a widescreen TV. I think it always assumes a 16:9 ratio. As @matthewpavkov suggested, best bet would be to get the tele to force the input into 4:3, but it's very model dependant if the tele actually supports that.

Update: I stand corrected, there seems to be a thread related to this exact issue on the MacRumors fourms http://forums.macrumors.com/showthread.php?t=658621 However, it seems that it stopped working on the 3.x firmware, so there's an even more hacky way of achieving it here. Sounds horrible though! http://forums.macrumors.com/showthread.php?t=811425