As I understand the way many parents who adopt do it, they explain that some children are gifts and some are chosen. Explain how a certain toy that was given to him is special, but another toy that he got to pick out himself is also special in a different way. So to you, he’s a gift little boy, but to your husband, he’s a chosen little boy. So while your husband isn’t his birth father, he’s going to be his adoptive father, and the United States government will make it official. (I’m assuming you’re in the US, if not, substitute the country you’re in, it’ll be just as impressive sounding.) I can’t imagine a four-year-old getting stressed over that.