A very ill-considered idea with kids that age, but I do think it could have been a different matter if it had been done in a high school class. The concept is blunt, okay, but it does bring home the reality of what slavery meant to people in those days and the hideous things that were accepted and taken for granted. And that seems worth highlighting because today, some people tend to mitigate this and say "It was so long ago, and after all you blacks were brought to America, the land of the free" and "Don't try to play the victim!"
There were a resonable number of people in that classroom it seems who were 'sold' as slaves, that reduces the risk of individual stigma I think. So I wouldn't condemn the idea in itself but 4th grade was certainly a too early level to put it into practice.
For comparison, I read about a suggestion intended for an educational theme day about world economy and divisions between rich and poor countries; this had been suggested around 1975, before the Asian tigers became really potent. The suggested activity - one of many - was that the students (age roughly 10 to 15 years) would be split up into a minor group representing the rich countries and a larger group, the third world. The "rich kids" would be treated to fairly mouth-watering food, ice-cream and attractively dressed tables; the poor kids would have to do with simpler and more meagre stuff, like, a bowl of rice, some veggies and bacon and, at the afternoon break, water and bread (maybe milk too) and they would have to be seated on large mats. I suppose there would be some way to secure that the kids didn't try to snatch food in the wrong place or sneak into the good queue. Now, that's a pretty blunt exercise and some parents would have jumped at it when they heard what had happened, but together with discussion it would have brought home a harsh reality,