I find the sizing on them to be really weird and honestly would not lie unless I can try them on. They didn't really work for a lot of my kids. I also bought myself up here at one point and they are horribly uncomfortable. I'm not even sure how the kids all wear them.
Might still be able to squeeze him into the kids size 4 but likely it would be too small. The men’s and women’s floafers are cut differently and not as good as the kids sizes. We switch to slides once they’re out of kids sizes in floafers which end at 4.