I'm almost clueless in this as well, but just for those who might know the answer ... is it just going to slip over the head, or is it going to have some kind of a clasp? If over the head, it probably needs to be close to regular size, cause I think a 4 year old's head is about 70-80% of what it will be when they are full grown ... that's just some odd thing that is sticking in my head ... could easily be wrong!!!