No need to prepare, in my opinion! They are only four! They are looking for engaged, curious kids. It helps if they are used to being with other adults and don't get too upset when they join the other kids. But my advice would be to not stress, the teachers are great and friendly and it's just a fun day for the kids. They are mainly looking for whether the kids would fit into to school, whether the focus is sports, arts, etc. It's also a chance for you to check out the school- works both ways!