A world expert on the effects of cold-water shock said that without fully functional lifejackets, the men would have stood little chance of survival.
Professor Michael Tipton, of Southampton University, said without such lifesaving equipment the men's chances of living through their ordeal would have been purely "down to luck".
Nobody knows exactly what happened on the fogbound night, in March 2009, when Glasgow men Craig Currie, 30, Thomas Douglas, 36, William Carty, 47, and Stephen Carty, 42, died after their small boat capsized in the freezing water.
But Prof Tipton, who was in the witness box for more than four hours as the available evidence was presented to him, said: "Their survival would have significantly increased had they been wearing lifejackets – working lifejackets as opposed to buoyancy aids. If you are not wearing a lifejacket, really it's down to luck."
The fatal accident inquiry at Oban Sheriff Court heard the men had not been wearing waterproof clothing, which would have given them extra protection in the water, which was about 5C at the time.
Procurator fiscal Craig Harris said that although Stephen Carty had been wearing a lifejacket it had not been inflated, had no cord mechanism on it, when found, to allow this – and was not fit for purpose as it had a hole in it.
He said: "There was a puncture, a 6mm long by 5mm wide hole, in the lifejacket."
The bodies of Mr Carty and Mr Douglas were found washed up on an island in the loch two months after the other two men and Prof Tipton surmised they had been separated from the boat and the other two men when it capsized.
He said the facts indicated something "fairly catastrophic" had happened to them in the water. He said: "Trying to piece it together, it would seem to suggest to me that these two of the gentlemen who were found some weeks later had an acute problem on the capsize, or the rapid sinking, of the boat."
The inquiry heard that Mr Carty and Mr Currie had been wearing light buoyancy aids unsuitable to save them in the emergency situation they faced in the water. Lifejackets are designed to keep the wearer's head above water and his airway clear if he falls unconscious, whereas a buoyancy aid is not.
Prof Tipton said: "Staying afloat in cold water is incredibly difficult.
"People cool about four to five times faster in cold water than in air of the same temperature."
He said people quickly become incapacitated by the shock effects of being in cold water, affecting the use of their limbs – making it difficult to move, swim, or even grasp hold of something which could save them – wiping out their ability to help themselves.
Alternatively, they could fall into unconsciousness.
A toxicology report showed a moderate level of alcohol in Mr Carty and low levels in the other three men. It also detected past use of cannabis by Mr Carty and Mr Currie.
The inquiry continues.