Part 4 of this book includes five chapters that touch on every populated continent on the globe. Because of this, there is no single explicit thesis statement that covers the whole part. However, there is at least one sentence that comes close to summing up Diamond's major thesis that he is trying to prove over the course of these five chapters.
This sentence comes at the end of Chapter 19. There, he tells us that European domination of Africa
was due to accidents of geography and biogeography--in particular, to the continents' different areas, axes, and suites of wild plant and animal species.
If we discount the fact that this sentence is specifically about Africa and Europe, we can use it as a thesis statement for the whole part of the book. This is particularly true of the first part of the sentence. In Part 4, Diamond is trying to prove that it was "accidents of geography and biogeography" that determined which people were to dominate the world and which were to be conquered.