If you are talking about chapters 4-9, then you are talking about the entirety of Part 2 in Harvest of Empire. This part, aptly named "Branches" does talk a lot about the differences in "migration and acclimation" of the different nationalities as you ask about in your question.
The literal "Branches" of the Hispanic "tree" are spoken of each in turn in Part 2 (and chapter by chapter): Puerto Ricans, Mexicans, Cubans, Dominicans, Central Americans, and Columbians. The first branch, Puerto Rico, is near and dear to the author's heart because it is his own heritage. Puerto Rico is a special case in that people from there are considered full citizens (although often looked down upon by whites). This is the reason why the author labels them both foreigners and citizens. Migration for Puerto Ricas is easier, however, due to their citizenship. The second branch, Mexicans, are considered "pioneers" of a different type. Not being citizens, like the Puerto Ricans, there is great hardship in coming over the border. There is either a mountain of paperwork or a huge journey in traversing the border illegally. Whether legal or illegal, Mexicans are then exposed to much racism on the other side of the US border. The next branch, the Cubans, are considered "special refugees" in that they are fleeing a communist country. Even though they are welcomed a bit more than the Mexicans into the main population, they still have their share of issues. Again, there are legal and illegal immigrants from Cuba, some of them fleeing through harrowing boat rides from Havana to Key West. The final chapters are about the Dominicans, Central Americans, and the Columbians and their unusual nuances about their migration and acclimation due to them having none of the extraordinary circumstances of the first three groups. Even though each "branch" has its differences, they all vow to combat both racism and division.
In conclusion, it is important to note the context of this second part called "Branches" that includes chapters 4-9. The author did many interviews from people of all different aspects from each Latin American nation. The interviews are included in the text and lend directly to the text's richness. The part about Puerto Rico is also quite special in that in contains the author's autobiographical information. Further, each "branch" of Hispanic America is shown to have both similarities and differences that enrich the entire culture as a whole.