The title of this chapter is “Farmer Power.” This tells you the general conclusion that Diamond makes in this chapter. His conclusion is that agriculture (which he sometimes calls “food production”) allows societies to become more powerful than societies that remain as hunter-gatherers.
The basic idea of this chapter can be found in Figure 4.1. It shows the ultimate causes behind European dominance. In Chapter 3, Diamond described all the reasons (which he boils down to the phrase “guns, germs, and steel) that the Spanish were able to defeat the Incas. In Chapter 4, he briefly lays out his argument as to why the Spanish had “guns, germs, and steel” while the Inca did not. His argument is that the people in Eurasia had agriculture before anyone else and, therefore, they became more powerful.
At the end of the chapter, he essentially sums up the message of the chapter. He says
Hence, the availability of domestic plants and animals ultimately explains why empires, literacy, and steel weapons developed earliest in Eurasia and later, or not at all, on other continents.