Police have been given more time to question a male nurse arrested on suspicion of tampering with medical records at Stepping Hill Hospital.
Victorino Chua, 46, was arrested on Thursday by police probing the poisoning of patients at the hospital in Stockport, Cheshire.
Detectives have obtained a warrant of further extension that is due to expire at 10.10am tomorrow, Greater Manchester Police said.
Mr Chua was arrested at his Stockport home on suspicion of unlawfully administering or causing a person to take a noxious substance, or poison, after it was found that medical records were tampered with on Monday and a patient given incorrect medicine.
Police stressed the suspect continued to be questioned only in relation to reports from hospital staff on Tuesday that medical forms had been altered overnight and a patient was given additional medicine.
The patient was monitored and subsequently discharged.
The suspect was held as it emerged a fourth death was being investigated.
The family of Glasgow-born William Dickson, 82, a former journalist, of Cheadle Hulme, Cheshire, who died on New Year's Eve, have paid tribute to him.
In a statement, they said: "Bill was a very kind and generous man with a great sense of humour.
"He was a hardworking and dedicated journalist, well respected by his colleagues.
"A loving husband, father and grandfather, he will be sorely missed by his family."
Police are investigating whether the poisoning Mr Dickson suffered at the hospital during the summer was a factor in his death.
His is the latest death to be examined by officers investigating the contamination of saline drips in June and July last year, causing patients' blood sugar to fall in "hypoglycaemic episodes".
The four fatal cases, along with Mr Dickson, are Tracey Arden, Arnold Lancaster and Derek Weaver who died shortly after their hypoglycaemic episodes.
All the deaths took place on wards A1, A2 or A3 - acute care wards caring for seriously ill patients.
Police said 20 patients are believed to have been deliberately poisoned.
The cases of 21 patients who suffered hypoglycaemic episodes are not being treated as suspicious.
Police have yet to establish a causal link between the deaths of the four patients and the contamination of their saline drips.
Mr Chua is one of 650 people already spoken to by police during their complex and long-running investigation at the hospital since the saline drip contaminations were discovered.