"They spent the night in near-freezing conditions - they were really lucky to have made it through the night," said the rescue helicopter paramedic who treated them.
The group were fishing off Kawau Island when their 6m aluminium boat began to sink half-way between Martins Bay and Motuketekete Island about 3pm on Tuesday.
The group swam nearly 2km to the uninhabited Moturekareka Island, but became separated on the way and Mr Preston disappeared just after dark.
He was wearing a lifejacket, but it was found floating in the water yesterday. This morning a search has resumed, although hopes of finding him alive were fading.
After three hours of swimming in water estimated to be 14C, the four made it to shore.
Two who had become separated found each other on Moturekareka but had to wait until low tide at 3.15am before they could walk to join the others on neighbouring Motuketekete Island.
Two of the men were in their 30s and one in his 40s.
Dressed in sodden jeans, jerseys, T-shirts and their inflated life-jackets, the four huddled together for warmth.
The alarm was raised after 9am yesterday when the wife of one of the men called the police.
The force's Eagle helicopter spotted them on the south side of Motuketekete about 9.50am.
Westpac rescue helicopter chief paramedic Barry Watkin said the four survivors were "very, very hypothermic" when the chopper found them.
"They were extremely lucky to be alive in conditions like that. We just covered them in blankets and warmed them up as soon as we could.
"They were pretty quiet when we got to them. They were very, very cold and I think quite shaken by what they'd gone through."
Mr Watkin said they were taken back to Stanmore Bay, where ambulances were waiting to take them to North Shore Hospital.
Eight vessels - including from the Harbourmaster, Coast Guard, Navy and police - and the Eagle and Westpac helicopters spent yesterday searching the area where the man was last seen.
"The visibility was good and we found a little bit from the boat - but unfortunately we found nothing," Mr Watkin said.
Coast Guard duty officer Mark Leevers said it was unlikely a person could survive another night in the freezing conditions.
He said the alarm had been raised so late because the group's communication equipment stopped working when it became wet.
"If they had been able to make contact with us as soon as their boat sank, this would have been a very different story ... It's just so important that people make sure they have mayday equipment."
By last night, the men taken to hospital were all stable.
One man was discharged yesterday afternoon and the other two were expected to be sent home today.
They were not up to speaking about their ordeal last night and a hospital spokesman said they were all "pretty shaken up".
The boy was transferred last night to the children's ward at Waitakere Hospital, where he will remain under observation until today.
North Shore Hospital emergency medicine specialist Dr Emma Batistich said the men and boy were in a "remarkably stable" condition when they arrived there.
"They had already been warmed up quite significantly by police and paramedics. They came in with normal temperatures," she said.
"We continued to warm them up and gave them [intravenous] fluids because they were still quite dehydrated."
Dr Batistich said she had not been expecting them to be in such a good condition. "I was surprised, given they had been in the water for a significant period and out overnight in very cold temperatures."
She said the men were doing "okay" emotionally, considering their crewmate was missing.
