The plot of a story is made up of five basic parts: exposition, rising action, climax, falling action and resolution. As part of the exposition, an initial inciting incident is brought into the story. This is an initial conflict that basically gets the ball rolling and starts to make things happen in the story. For The Giver, the first important conflict is that Jonas is chosen to be the new Receiver of Memory.
Since you ask for four sentences, I will break them into the other types of plot after the initial conflict that lead to the resolution.
The rising action of the story is Jonas’s first experiences as a Receiver of Memory, when he learns that the community’s memories will now be stored with him and he begins to see things he has never known before, such as color.
The climax of the story, when the conflict comes to its most serious point, is when Jonas watches the Release of the newborn twin and he realizes that Release means death and the community kills anyone who does not conform.
The falling action of the story is when Jonas learns Gabe is to be released and decides enough is enough, and he and The Giver plan his escape to Elsewhere with Gabe.
The resolution of the story is a little tricky (because this story has an ambiguous ending), but Jonas and Gabe either reach Elsewhere or they die in the snow.