Three are being cared for at Karolinska University Hospital in Solna and a fourth has been sent to Åhus in Denmark for treatment.
Two of the critically ill patients are from the Stockholm region, one is from Östergötland, while the fourth is from Gästrikland.
All are described as being under 30-years-old, senior physician Stefan Engkvist told the TT news agency.
Two of the patients at Karolinska are being cared for at the ECMO unit and the third has also been put on an artificial lung apparatus but is being kept in a separate intensive care unit.
“There is nothing out of the ordinary with their condition,” Stefan Engkvist said but added:
“I mean. They have no lung function. That is very serious. But as soon as they receive treatment the condition is stabilized. You feel pretty good when you are on ECMO.”
Engkvist explained that it is difficult to draw any conclusion from the four cases which have emerged despite the fact that the flu has not yet become widespread in Sweden.
“No. One can not develop statistics from four people,” he said and added that no one knows exactly how badly Sweden will be hit.
He explained that experience from other countries has shown a lower proportion of seriously ill patients and a lower level of fatality than a normal seasonal influenza.