Medics appeared to remove one person on a stretcher but hid the person's identity behind multiple white umbrellas.
Navy SEALs arrive near cave where 12 boys and their football coach have been trapped since June 23.
Footage released by authorities showed groups of soldiers carrying what appeared to be two of the boys on stretchers into the back of waiting ambulances which then sped off for a nearby hospital.
Authorities have said helicopters were ready to take cave evacuees to a hospital.
Narongsak Osotthanakorn confirmed during a press briefing that four of the stranded party had been rescued and sent to Chiang Rai Prachanukroh Hospital.
The first boy made it out at 5.31pm (11.31am United Kingdom time), and at 7.47pm (1.47pm) the Thai navy SEALS announced: 'Fourth Wild Boar out of the cave!' The youngsters were assessed by medics at the cave entrance before being taken to hospital.
The day's operation started at 11 a.m. after divers finished reinstalling oxygen tanks and doing rope checks along the exit route that includes flooded narrow passageways. He said it could take several days for doctors to replenish the nutrients the boys lost during their cave ordeal. While there was no immediate word on their condition, Dr. Ashton said doctors will likely conduct a battery of tests on the boys to evaluate not only their physical condition but their mental condition as well. The ordeal has riveted Thailand and made global headlines, and the search and rescue operation has involved worldwide experts and rescuers.
It is believed they have been able to make the last part of the journey on foot.
In an global rescue mission that has gripped the entire world, at least four members of a boys' soccer team were freed on Sunday from a cave in northern Thailand where they had been trapped for more than two weeks. The most touching note came from one whose name was not clear: "I'm doing fine, but the air is a little cold, but don't worry". Officials did not immediately comment.
He also told reporters divers need to place more air canisters along the underwater route to where the boys and their coach have been trapped.
Australian Foreign Minister Julie Bishop, in an interview with CNN affiliate Nine News Australia Monday, said that the remainder of the boys were also expected to be extracted in batches.
But the survival of those remaining in the cave is far from guaranteed with extraction efforts put on hold until Monday morning to allow rescue divers time to resupply.
The governor announced a "back-up plan" to dive out the 12 boys and their football coach should the monsoon rains resume before other options could be used to rescue them from a muddy rock shelf in the Noen Nom Sao cave, which is situated 2.5 kilometres inside the larger Tham Luang cave system.
He said the healthiest were taken out first, and the next phase of the operation would start in 10-20 hours. Musk said based on "feedback from Thailand", the engineers made a decision to build a "tiny, kid-size submarine" using a large silver tube meant to be affixed to a SpaceX Falcon 9 rocket.
Musk posted videos on Twitter of the aluminum sub being tested Sunday at a swimming pool.
His parents, along with other families, have maintained a constant vigil at the site since the boys became trapped.
A spokesman for Boring Co. said on Sunday that the company has four engineers who are "offering support in any way the government deems useful".
Earlier Sunday morning, Trump tweeted that the USA government is working with Thailand's government in the rescue operation.
In a tweet Sunday morning, U.S. President Donald Trump said, "The U.S.is working very closely with the Government of Thailand to help get all of the children out of the cave and to safety".
The area outside the Chiang Rai hospital was cordoned off by police.