Don't get too excited, "Donald" is just making a wordless cameo in the next chapter. However, you will find out what the relationship is between him and Nina at this point, so many years after their high school drama. Also, his future occupation will play a part in the plot, though in story he hasn't even made it to med school yet.
In other news, I can tell you that chapter 5 will end with a scene of Unresolved Sexual Tension between two characters. This does not involve Mike or Don. You can probably figure out who one of the characters is, but you're probably going to think I'm incredibly weird when you find out the other one. (Maybe not. A non-Simmer would most likely think so, though.)