Another baby is being treated for an infection which has killed three newborns at a hospital.
A bacteria called pseudomonas caused the deaths at the Royal Maternity Hospital in Belfast.
Health officials in Northern Ireland also revealed that a different - and apparently unlinked - strain of the infection claimed the life of another baby in Altnagelvin hospital in Londonderry last month.
In the wake of the three deaths at the Royal Maternity, a spokeswoman for the Department of Health, Social Services and Public Safety (DHSSPS) said four other babies were found to have the infection in the neonatal unit.
One is undergoing treatment, two have been treated and already recovered, while the fourth made a recovery from pseudomonas but subsequently died of unrelated causes.
The DHSSPS spokeswoman said the baby still undergoing treatment was doing well. Other vulnerable babies have been swabbed for signs of the infection, while a helpline has been set up for worried relatives.
In regard to the death in Derry in December, officials said the strain of the infection, which was subsequently eradicated within the hospital, was different to the one that has hit the Royal's maternity unit and there was no evidence that the outbreaks were linked.
The bacteria can cause infections in the chest, blood and urinary tract. The neonatal room at the Royal is being emptied after the outbreak and will undergo a deep clean.
Stormont Health Minister Edwin Poots has received regular briefings from health officials as the situation developed. "It is important that the parents and families of the babies affected are given all the support they require," he said.
Belfast Health and Social Care Trust chief executive Colm Donaghy said it would be carrying out a full investigation into whether anything else could have been done. The first death happened on January 6 and the second on January 13. The third infant died late on Thursday night.