Sick baby, need help

I have hedgehog babies that are a week old. She had five, one died at day four. With the other four, three of them seem to be in great health. One however is half the size of the other three, seems to be warm and wiggling just as much as the others. But he/she is way to tiny compaired to the others. This is my first hedgehog and like many others she had babies right after I got her. I dont really know anything about them yet just what Ive googled and read on here. Somebody please tell me what I need to do before this little guy dies too. I have (tried, dont know if it worked) attacked a picture of the runt with one of the big ones. I also placed a quarter next to them to give you an idea of how big they are.