I have used a similar red/yellow/green system in the past and it was pretty effective. I used a yardstick which I just colored with markers. Everyone started on green every day, so green was the largest section. Sorry I don't have a picture!
I think the most important part was for student to see a visual consequence for their behavior, positive or negative. As tsara said, the trick is for some students to understand, "I hit so-and-so and that's why I'm on red." I think for this system to work for 4 year olds, you really have to review behaviors & consequences at each opportunity.
I did 2 things that were very meaningful to the kids: I recorded in their folder everyday what color they were on & why. Also, I added purple to the top of my yardstick for exceptionally helpful, caring, positive behaviors. Kids & parents loved being on purple & I recorded those actions just as well as the yellow & red ones. The specific examples really made a difference.
I did have to make a clothespin for each student. I had 2 classes, so I used different colors of the plastic clothespins for each class & wrote their names on with a sharpie. 2 classes makes everything with organization difficult! Good luck!