Authorities say the 18-year-old was found after a hiker heard a woman yelling for help. Even after a rescue crew heard Jack's screams, it took them 90 minutes to get to her.
"First she saw me, and I could not see her, and so I was screaming, yelling across the canyon, 'Can you see me?' She said, 'Yes, I see you.' I could not see her yet because she was on top of a rock behind a bush," said Los Angeles County Sheriff's Reserve Chief of Rescue Mike Leum.
Jack was found wearing shorts, a sweatshirt and socks. She was airlifted to UCI Medical Center where she was evaluated and was listed in good condition, UCI Medical Center spokesman John Murray said in a late afternoon press conference.
Murray said she suffered no major internal injuries. She did have scratches and bruises on her lower body, and she was dehydrated and hypothermic. Authorities also say Jack suffered an arm injury and could only wave with one arm when rescuers were trying to locate her.
Murray said she would be in the ICU for a few days of observation.
"It's been a long four days for her out there, very dangerous. She wouldn't have made it much longer, she is really, really lucky," said Los Angeles County Sheriff's Deputy Jim Moss.
A reserve deputy who was aiding in Jack's rescue suffered a head injury when he fell 60 feet down the canyon. He was transported in serious conditon to Mission Hospital in Mission Viejo. He was expected to make a full recovery. Orange County Sheriff's Lt. Erin Giudice described the deputy as awake and alert, and able to move his arms and legs. The deputy's name has not been released.
Murray said Jack's parents were at the hospital, and they were thankful to all who participated in the search and kept their daughter in their thoughts and prayers.
Jack's friend, 19-year-old Nicholas Cendoya, was found alive just before sundown Wednesday. He was located in thick brush by a hiker who was unrelated to the search. He was rushed to Mission Hospital, where he is recovering in intensive care. His condition Thursday was said to be serious but stable.
The search for the two young hikers began Sunday when they called 911 for help around 8:30 p.m. and told dispatchers they were hiking on a trail but couldn't find their way back down the rugged terrain. They also said they ran out of water. Their cellphone batteries died before police could trace a signal.
The search on Thursday focused on the area where Cendoya was found, which is about a half-mile south from the Trabuco Canyon trail head. Authorities say both teens were found close to one another in an area with high brush and very rugged terrain.
One problem crews faced was thick brush. Officials said even when the rescue helicopter was over Cendoya, rescuers briefly lost sight of him because the brush was so thick.
Cendoya's family said they're very grateful that he was found alive.
"We just want to thank God, and we're very happy. Thank you very much to everybody that volunteered and helped out," said Patricia Sanchez, Cendoya's cousin.