Reddit reviews The Red Thread: An Adoption Fairy Tale

I would start with the idea that families are all different - there are many different ways to be a family. Todd Parr's The Family Book or Families, Families, Families! are both fun ways to introduce diverse family structures that would be appropriate for a 4yo. Both include families with children who look different from their parents, which could be a good way to bring up your friends' family. There are also some good picture books that focus on interracial adoption specifically, such as All Bears Need Love, Horace, A Mother for Choco, Over the Moon, and The Red Thread.