To prevent spoilers, I will just say that the fourth book continues with the theme of the high cost of love. It creates a character from the first book and weaves a story around the search for her missing son. In the midst of her story we find out what happened to Jonas and Gabriel.
It does not have the same "wow" factor as the first book, but for those who need a satisfying conclusion to the series, this should fit the bill. The interim books (Gathering Blue and Messenger) are only loosely connected to Book One, but Son is THE sequel to Giver. (You can even skip the middle two books and not miss a whole lot.)
Some critics complained that the first book stretched the limits of belief. I didn't feel that way, but there were definitely moments in Son, when I was wrinkling my forehead and saying, "Really?" Still, it is worth reading if you like a compelling story of the pain and pleasure of loving deeply.