According to Guns, Germs, and Steel, what are the four sets of "ultimate factors" cited by Diamond that tipped the advantage to European invaders of the Americas?
The answer to this can be found in Chapter 18. Specifically, it can be found on p. 370 in the paperback edition of the book.
First, though, please note that while you ask for four sets of ultimate factors, Diamond only lists three. As he says,
…we have identified three sets of ultimate factors that tipped the advantage to European invaders of the Americas…
So, let us look at these three factors. We will also look at one more that he calls “speculative.”
First, Eurasia had human beings inhabiting it for much longer than the Americas did. People did not arrive in the Americas until millennia after they lived in Eurasia. That gave Eurasians a head start.
Second, Eurasia had better food production. We have already seen that the Americas lacked large mammals that could be domesticated. We have seen that they did not have as many good plant species. This all meant that agriculture was more effective in Eurasia.
Third, it was much easier for crops and technology to diffuse in Eurasia. That landmass has a long east-west axis. The axis of the Americas is north-south. In addition, the Americas have many features, like jungles and deserts, that make diffusion difficult.
The last, and “more speculative” of these is that populations of people in the Americas were split up into virtual islands. That is, people in the various societies could not come into contact with one another to form larger communities.
These are the ultimate factors that Diamond identifies.