Part Four, Sal's and Dean's final journey, begins with Sal telling the reader that he came into some money by selling the novel he had been working on in the previous parts of the book. It is spring, and Sal again feels the need to travel. This time, however, he sets out without Dean, leaving him at his job at a parking garage in New York-still living a domestic life with Inez in New York. Before he leaves, Sal and Dean talk about their lives in New York. Dean seems to have found some happiness with Inez, a woman who lets him get his "kicks" with a "minimum of trouble." Dean tells Sal they should eventually grow old and be bums. After a final Sunday afternoon in which they play ball with neighborhood boys, Dean repays the fifteen dollars he has owed Sal's aunt from the speeding ticket she paid, and she feeds the two a big meal during which she tries to compel Dean to stay married and take care of his children. The two say goodbye to each other. Sal tells Dean that he hopes they can one day settle down with their families on the same street-an image of domesticity they had been running from for the whole novel.
Sal takes a bus to Washington, ranging about the South, visiting Stonewall Jackson's grave, then the Midwest, and finally gets to Denver with a friend he makes on the bus, Henry Glass, a young kid just released from prison. Sal takes Henry under his wing and escorts him to Denver, where Henry's brother has a job for him that will help him stay out of trouble. In Denver, they meet up with old friends Tim Gray and Stan Shephard. Stan wants to follow Sal to Mexico, and the two agree to travel together.
Sal stays with his old friends, Tim and Babe Rollins, for a week in Denver. They party and visit the jazz clubs and make preparations for the trip to Mexico. As he gets ready to leave, Sal gets word that Dean bought a car and is on his way to Denver, supposedly to drive Sal to Mexico. At that moment, Sal has a vision: Dean is the Shrouded Traveler, a "burning shuddering frightful Angel," blazing across the Midwest leaving death and destruction in his wake. Sal is uncertain about Dean's arrival, fearing for the children he is leaving behind and the money they will not get because he spent his savings on a car. Sal realizes that everything about the trip must now change with Dean's arrival.
When Dean does finally show up, they rearrange their plans for Mexico and Sal admits that he feels all right with Dean's arrival. Sal adds that he cannot help but follow him wherever he goes. They spend a night in Denver at the Dunkels' house, reunited with their old friends. The Dunkels talk about their plans for the future, going back to school and settling down with family. Dean's madness, for the first time, seems out of place at the party. He attempts to entertain and infuse the party with wildness, as he had done in the past, but his presence only makes his old friends uncomfortable. The group migrates to the Windsor Hotel bar, where Dean and Sal get "fumingly" drunk. Sal breaks one of his fingers punching a door but does not realize it until the next day.
The next day they map their trip to Mexico, the "magic south." Dean declares that this is the trip that will finally take them to "it." Stan says tearful goodbyes to his overprotective family, and the rest say goodbye to their Denver friends. Sal, Dean, and Stan take off for Mexico.
Three miles outside of town, a bug stings Stan in the arm while they are driving, causing it to swell. They decide to stop at a hospital to get Stan's arm checked out. As they drive, they recount the stories of their lives, Dean instructing Stan to "deal with every single detail." They drive through Texas, taking in the sights of the prairies and plains. Stan relates his travels in Europe, and soon they are rolling into San Antonio, where they stop at a hospital for Stan's arm. After he gets a shot of penicillin, they go with Dean to check out the pool halls of San Antonio. Dean says he is high on the air of San Antonio.
They drive the rest of the way to the Mexican border by Laredo, Texas. At three in the morning they cross the border, where they discover that Mexico "looked exactly like Mexico." Dean and Sal are in awe of the men in straw hats lounging before battered storefronts. The border police check their baggage, and they exchange their dollars for pesos, excited to finally be in Mexico.
Sal's and Dean's final journey, in Part Four, takes them to Mexico to truly experience the marginal culture that they have expounded about and idealized throughout the novel. It is not African American culture, but it is a subculture all the same, even though for the Mexicans, their own culture is itself the dominant one. That is the point: finding a society where people can do what they want without worry.
The goodbye that Sal and Dean share in New York illustrates the ironies of their carefree choices. Sal says goodbye with the hope that the two will one day settle down with families into a quiet domesticity, the kind of life the two have been rejecting since their first travels. Dean, meanwhile, hopes that he and Sal will one day grow old together as bums, dropping completely out of society, not interfering with anyone and not being interfered with by anyone. As this new journey begins, Sal begins to confront his growing maturity (after all this time) to become distrustful of Dean's lack of conventionality. Yet, Sal is the one who is on the road.
Sal's budding maturity is seen further as he becomes a brief father figure to Henry Glass, the young ex-convict on his way to a job in Denver. Glass is the next generation-is it better to advise him to be a beatnik or not?
When Sal learns that Dean is coming to Denver, supposedly to drive him to Mexico, Sal has a vision of Dean as the Shrouded Traveler from Part Two. In part, this vision instills in the reader the kind of awe and legend that Dean's own friends felt for his arrival. In another sense, it is Sal's last apprehension about once again becoming overwhelmed and sucked into Dean's madness, a state he once longed for but now is not so sure he wants to participate in. Yet, when Dean arrives, Sal forgets his apprehension and they plan their trip to Mexico, a trip they believe will finally illuminate "it" for them.
Sal again feels a kind of momentary separation anxiety as he watches Tim Gray recede in the distance, just as he watched previous friends recede. He compares the city of Denver to the sinking city of Atlantis. This section of the novel, the closing journey, is filled with apocalyptic imagery, and Sal compares himself to the biblical wife of Lot from Genesis, looking back on the destruction of Sodom and Gomorrah. They have escaped the destruction of the evil cities, but Lot's wife looks back wistfully; her place is there.
As the group cross the Mexican border, Sal's sense of doom is replaced with the rush of the present. He describes Laredo, Texas, as the dregs of America, not just because it is one of the geographically lowest or most southern points in America, but also calling to mind the night in the Detroit theater in which Sal compared himself to the garbage of the place. Once over the border, they spot the Mexican culture that surrounds them, so they can easily forget such feelings as left behind in the United States. Instead, the excitement of travel resumes.