I'm working on a space invaders type setup, needed enemy sprites to go left to right, so I gave them the bullet behaviour.
So for example: "system: from start layout.. Action: Sprite- set bullet enabled.
Action: Sprite- bounce off objects enabled.
so it would go left to right and the barriers at each side of my setup would act as buffer to keep it going back and forth along that same straight line.
in your case, might help note the positons on left and right side and set it as move forward to 1234.
and then.. (when at 1234, move backwards to 4321.)
just be careful keep an eye on any other actions you're giving it that might contradict the one you want it to do.