The sequence of Gulliver's Travels is not particularly important except as it informs Gulliver's personal character change from a typical, nationalistic Englishman to a recluse who believes horses to be superior to humans:
Have not I the most reason to complain, when I see these very Yahoos carried by Houyhnhnms in a vehicle, as if they were brutes, and those the rational creatures?
(Swift, Gulliver's Travels, gutenberg.org)
The four parts of the story were written out of order; the first two parts of the story were written together, and then Part 4 was written before Part 3. This may have allowed Swift to figure out his ending before working on the far more diverse and complicated adventures in Part 3; while the other three parts have only one major culture and civilization to explain and contrast with England, Part 3 has five separate cultures, one of which (Japan) is based in reality. This gave Swift more scope to satirize aspects of England, but also made it harder to get into deep details on any one of the cultures; Gulliver has less time to react and try to explain himself, and so this section has less effect on his character arc than the other three. This could also be a reflection of the out-of-order writing, since Swift had already closed
Gulliver's story arc and so wasn't worried about giving him major intellectual challenges.