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Inside a room in the Ho Chi Minh City Department of Justice on March 15, officials nervously paced back and forth in anticipation of an adoption ceremony unlike any they'd ever known. Then, at 10 a.m., a door swung open and Angelina Jolie appeared, looking reserved and carrying her 5-year-old son Maddox in her arms. But all eyes quickly shifted to the boy of the hour: a quiet 3-year-old dressed in a white T-shirt, gray pants and a pair of red-framed sunglasses.

Accompanied by a Vietnamese assistant, "he looked shy and dazed," says Vinh Huy, a reporter for the Ho Chi Minh City Law newspaper who observed the scene. In contrast, a rambunctious Maddox "ran here and there discovering things." After a series of document signings, the department director handed finalized paperwork to a smiling and visibly relieved Jolie, who vowed to return to Vietnam, saying she wanted her new son "to be proud of his country," recalls Huy. "She said it was an important day in her life."

With that, Jolie, 31, became a mother for the fourth time—and the Vietnamese orphan with the sweetly shy smile and the big brown eyes officially began his new life in the world's most famous multicultural family. Newly named Pax Thien Jolie—Pax is Latin for peace; Thien traditionally means sky in Vietnamese—he joins big brother Maddox (adopted from Cambodia), along with sisters Zahara, 2 (adopted from Ethiopia) and 10-month-old Shiloh, the daughter born to Jolie and her partner, Brad Pitt, 43, last May. Already Jolie is talking like a proud new mother. "You can imagine what courage it takes to be in all new surroundings, with new people and a new language," she tells PEOPLE. "He is very strong."

And Jolie is committed to making his transition as smooth as possible. "It will take him a while to realize he has a family," she says, "and that his new life is permanent and that it won't keep changing." To that end, Jolie told the Law that for the time being she has no plans to resume work. "I will stay at home to help Pax adjust to his new life," said the actress and activist, who was set to return to the U.S. after obtaining Pax's passport and visa in the Vietnamese capital of Hanoi. "I have four children now, and caring for them is the most important thing for me at the moment. I am very happy to be their mother."

As for Dad, because Vietnamese regulations don't allow unmarried couples to co-adopt, Jolie adopted Pax as a single parent while Pitt remained in L.A., where he is filming The Curious Case of Benjamin Button. "He has specific days on the movie that couldn't be changed or production would run over," says his rep. But Jolie still made sure to bring a welcoming committee: Joined by Maddox and Zahara—Shiloh has been on the Button set every day with her father—the new mom used her first few days with Pax to begin gently bonding with him and to allow her other kids to do the same. "We are slowly beginning to build his trust and bond," Jolie says, "but it will feel complete only when we are all together."

One immediate challenge: the language barrier. Pax, who until now has only known life inside the simply appointed Tam Binh Orphan Center on the outskirts of Ho Chi Minh City, speaks only Vietnamese, while Jolie has learned a few phrases in his native tongue. Dr. Jane Aronson, a Manhattan pediatrician and international adoption expert who became a friend of Jolie's following Zahara's adoption, notes that Pax's years in an orphanage may initially slow his language abilities and that learning English will likely take several months.

A soccer-loving boy who has a tendency to erupt in laughter one moment and then suddenly turn shy—as he did in his first one-on-one meeting with Jolie at the orphanage on March 14—Pax had not been told he was to be adopted until that day, for fear that "we might put pressure on him and make him more frightened," says Bui Thi Thanh Tuyen, his primary caretaker at the orphanage. When an overwhelmed Pax began to cry after meeting Jolie, she quickly showed her maternal instincts: "She told me she understands—that it's normal for all young children to be scared," says the orphanage director, Nguyen Van Trung. Later, after Jolie had taken Pax aside to comfort him, "he was very cheerful and happy," says Trung. "He even played with his new brother Maddox."

A new brother, new sisters, celebrity parents and a globe-trotting lifestyle—it's hard to imagine how the world could change more dramatically for Pax, who was one of 326 children at the orphanage. Abandoned as a newborn by his birth mother at a local hospital, Pax—then called Pham Quang Sang—spent his days in the facility's structured environment: Toddlers rise at 6 a.m., when they brush their teeth and wash up before eating a typical breakfast of bread or rice porridge. Then caretakers bring the children outside to play before engaging in group activities like singing and drawing. At 8:30 p.m. the lights go out and the children are tucked into their rows of small, plain beds. Gesturing to Pax's now-empty bed near the corner of the children's bedroom, Tuyen says, "he was here, and now he's gone. Everyone is sad since he left. We have mixed feelings—happy, proud of him, missing him."

Suddenly out of the orphanage, "the big issues will be intimacy and teaching Pax about family life," says Aronson, herself the mother of an 8-year-old son adopted from Vietnam. "He's in the mind-set of, 'I'm in the pack, and I can't always get what I want.' Now he'll learn that his wishes will be respected."

Contrary to some reports, Pax's adoption was not fast-tracked, nor did Jolie, who requested a healthy boy—many of the children at Tam Binh are HIV positive—donate money to the orphanage in order to curry favor. "Things just fell into place," says Heidi Gonzalez, the Vietnam adoption program coordinator for Adoptions from the Heart, the Pennsylvania-based agency that arranged Pax's placement. "Angelina contacted me [in late fall] as I was looking for a home for Pax."

After being given information about the boy, Jolie followed up with a November trip to Vietnam, a country, along with all of Southeast Asia, where Jolie has said she feels "at home." Pitt and Jolie visited the Tam Binh orphanage, "but she didn't meet with [Pax] individually," says Gonzalez. "She didn't want to single him out. So instead she met with all of the toddlers and brought them gifts." But even from a distance, Jolie clearly clicked with the "friendly and very shy" little boy, says Gonzalez: "It was like it was meant to be."

Clearly his new family agrees. While Pax adapts to life in the Jolie-Pitt household, the group will likely forge new ways to understand each other. "It's eye contact, it's touch, it's looking, it's being attentive," says Aronson. "The advantage is that Angie has a son who will be the big brother and will be there to help Pax not be afraid. Maddox will be the great teacher here. He'll be the great love object for Pax." No doubt there will be plenty of love to go around. Pax's new parents, adds Aronson, "are both bright people who love children and want to do the best by their kids ... It's just a fantastic win-win situation."

Get even more news and photos on Angelina's growing family at PEOPLE.COM/PEOPLE/ANGELINA_JOLIE

  • Contributors:
  • With Kay Johnson/Ho Chi Minh City,
  • Do Minh Thuy/Ho Chi Minh City.