What event did Harry Potter take part in during his fourth year?

The Triwizard Tournament

Rubeus Hagrid tracks Harry Potter down and tells him that he is a wizard. One event in Harry Potter and the Sorcerer's Stone is when Harry sees the Mirror of Erised.

a major event in harry potter would be when he defeated he who must not be named! (Voldermort)

The big event in the first year of Harry Potter is that Voldemort, or You-Know-Who, was going after the sorcerer's stone. This would have made him immortal.

In Harry Potter and the Deathly Hallows The series pretty much leads up to the death of Voldemort. You could also say the Final Battle is the main event, or the search for the Horcruxes.~Zaara

There is no mention of any such event.

One main event in "Harry Potter and the Chamber of Secrets" is the house-elf, Dobby, coming to visit Harry to warn him not to go back to Hogwarts. Harry, Ron, and Hermione making and taking polyjuice potion is another main event. Harry finding the Chamber of Secrets and rescuing Ginny is another main event in the story.

Harry Potter and the Goblet of Fire is about the triwizard tournament, which was hosted by Hogwarts School. Harry was selected as a champion and had to compete, he ended up a joint winner of the tournament.

Well just about any event in the story is important to the plot. The arrival of Haggrid or perhaps Harry getting his want from Olivanders.

The climax is when Harry Potter has to fight Professor Quirrell and Lord Voldemort. Quirrell tries to take the stone from Harry, but suffers severe injuries whenever he touches him. Harry uses this to his advantage by putting his hands on Quirrell's face. Dumbledore arrived as Harry passed out and managed to save the Stone. Quirrell died of his injuries.

The Chamber of Secrets being opened and Harry Ron and Hermione trying to discover who opened it and where it is.

Voldemort killed his parents.

There were a few major moments for the shaping of Harry in the Sorcerer's Stone. However, the arguably most changing was his discovery of the Mirror of Erised.

Harry Potter goes back to Hogwarts and finds out that there will be a tournament called the Triwizard Tournament. Two other Schools show up to compete in the tournament and a representative of each school is chosen to compete. After each School's representative is chosen by the Goblet of Fire, it does something it doesn't normally do. It spits out another name. Harry Potter becomes the fourth competitor alongside Hogwarts other competitor Cedric Diggory. Eventually…

Harry Potter, Ron Weasley and Hermione Granger go through a series of challenges to stop Professor Quirrel and Lord Voldemort from stealing the Philosopher's Stone. Harry then has to face both Quirrel and Voldemort alone which results in the death of Quirrel.