This is when I consider the series to turn dark. This is the first book in which a character with whom the readers are familiar dies. The Triwizard Tournament is held at Hogwarts during Harry's fourth year, and visitors from two other wizarding schools are invited to stay at the castle for the year to compete in the Tournament. Although Harry isn't old enough to compete, his name is produced from the Goblet of Fire, and he must compete, as this represents a binding magical contract. Obviously someone has put Harry's name in the goblet to try to bring harm to him, as the Tournament is highly dangerous and people who don't have enough magical training could die while completing the three tasks. After a year of Harry surviving Tournament tasks, fights with friends, and girl troubles at the Yule Ball, he competes in the third and final task. But someone has rigged the game, and as Harry completes the last task for the win, he is magically transported to a graveyard, where Lord Voldemort is resurrected. After being tortured for a couple dozen pages, Harry manages to escape the Dark Lord and get back to the Hogwarts castle. In the end, only Dumbledore and a few of Harry's friends believe his story of Voldemort's return. The Ministry of Magic refuses to believe him, thus setting up the premise of book #5.
Wow, this is a long book. I'd forgotton how long these get toward the end of the series. It's so long that the movie really couldn't possibly include everything unless they made a 5-hour movie. There were many plot developments and a few sub-plotlines that were completely left out, so it was exciting to read again since I couldn't remember everything.
Once again, one of my favorite parts of this series is following the characters as they grow. Rowling seems to think that in their fourth year (when Harry is 14 years old), the students of Hogwarts must have matured enough to think about the opposite sex. This allows for a few comedic scenes where Harry and Ron worry about asking a girl to the Yule Ball, and a few painfully embarrassing scenes (at least for those of us who remember middle school) where feelings between Hermione and Ron are suggested but ignored. Rowling also seems to think that at this age, the students are mature enough to handle more Dark things. Harry's friend/competitor, Cedric, dies, and Lord Voldemort returns. The scene where Voldemort tortures Harry with the Cruciatus Curse (an Unforgivable Curse that causes intense pain in the recipient's entire body) is pretty disturbing. I wouldn't want my little sister reading that before bedtime.
The series is written in the third person, but limited to Harry's point of view. As is usual in the case of books vs movies, the book does a better job of describing how terrible the experience of coming face-to-face with Voldemort is and all the emotional issues Harry has, mostly just because Rowling has the advantage of peering into Harry's thoughts. I've already started onto the fifth book, which picks up a few weeks after Harry leaves Hogwarts, and Rowling describes his issues from such a traumatizing experience and how he has regular nightmares about it. Harry's life gets pretty dark from here on out.