Your question, "What are four Key Concept Syntheses in Markus Zusak's I Am The Messenger?," may actually be a bit misleading because Key Concept Synthesis is merely an analytical process, a way of looking closer at any text. We actually use Key Concept Synthesis to identify main ideas within a text and understand them further by synthesizing the main ideas with our own knowledge. It seems what you really mean to be asking is, "What are four main ideas in Markus Zusak's I Am The Messenger, and how does one apply Key Concept Synthesis to identify and further understand these main ideas?"
Key Concept Synthesis is a term that has been given to a method students can use to improve their reading comprehension. Using the process, students identify key concepts within a text, explain those concepts in their own words, and then relate those concepts to other ideas based on their own knowledge bank (MetaMetrics, "The Lexile Framework for Reading in Action"; WritingFix, "Key Concept Synthesis"). In terms of non-fiction, when implementing Key Concept Synthesis, students will search for main ideas within the text by focusing on text divisions, like chapter titles and headings; looking carefully at any visual aids, like pictures and graphs; and searching each paragraph for topic sentences. In terms of fiction, an author's main ideas will be found in the author's themes, so to implement Key Concept Synthesis, readers will search for a story's or novel's themes. A theme is an idea that an author repeatedly portrays or refers to throughout his/her work; a theme will also be universally applicable (Literary Devices, "Theme Definition"). For example, one theme in Jane Austen's Pride and Prejudice is prejudice among social classes, which even today is universally applicable. Hence, as you search for main ideas within Markus Zusak's I Am The Messenger with which to apply Key Concept Synthesis, you will be looking out for any of Zusak's themes. As we are limited in space, below are a few ideas to help get you started.
I Am The Messenger is essentially a story about bravery and heroism. The young protagonist Ed Kennedy is an ordinary cab driver who finds himself getting mixed up in stopping an armed bank robber from leaving the scene of the crime. The robber drops his gun and tries to get away in a stolen, dilapidated vehicle, and Ed manages to pick up the gun and stop the robber from making his escape. Ed finds himself being reported on front pages of newspapers as a hero though he considers himself to be only an ordinary person and not the hero type. After being reported as a hero, he finds himself mixed up in some sort of game directed by an anonymous person in which Ed is given playing cards with names and addresses on them, provoking Ed to, in some way or another, rescue the individuals named on the cards. For example, Ed rescues a grieving war widow from loneliness, encourages an athletic teenager in finding her inner joy and beauty as she runs in her track meets, and eventually rescues a family from the abuses of a drunken father, among many other things. Hence, we can say that one of Zusak's themes is heroism, or exactly what it means to be a hero. Once you've figured out that theme and listed the details that help portray the theme, like the details named above, your next task is to either explain why the theme is important or to relate the theme to other things you already know. For example, you might think about exactly how important it is for everyday people to be able to view themselves as being able to accomplish the same sort of heroic deeds as Ed accomplished. Would it make the world a better place if every one tried to act as Ed did? How many people in the world do you think actually do live their everyday lives acting as Ed did?
It's also very important to notice that Ed is characterized as a very average person, maybe even below average. At 19 years old, his level of education extends no further than past high school; he is destined to live his life as a cab driver; and, he has relationship problems. Hence, Zusak seems to be juxtaposing our standard definition or understanding of heroism with an ordinary person. Just as Zusak seems to be asking what it means to be a hero, he also seems to be asking exactly what it means to be an ordinary person or if ordinary people even exist at all. Hence, we could also say that a second theme in the novel is ordinary people, or more specifically, there is no ordinary person because all so-called ordinary people are capable of great things.
From there, you would continue exploring important ideas in the novel to find your other themes and apply Key Concept Synthesis to those themes by relating them to your own knowledge. The two sources provided, both above and in links below, also contain graphic organizers you can use to both list your key concepts, or themes, and analyze your concepts using your own knowledge.