Police are investigating the deaths of two more patients at the hospital linked to allegations of sabotage.
The patients were an 83-year-old man and an 84-year-old woman, Greater Manchester Assistant Chief Constable Ian Hopkins said.
Detectives are continuing to question a nurse arrested on suspicion of the murder of three patients at Stepping Hill Hospital in Stockport.
Rebecca Leighton, 27, was arrested yesterday at her home a mile from the hospital where she worked.
Tracey Arden, 44, George Keep, 84, and Arnold Lancaster, 71, are thought to have died following treatment at the hospital.
The 84-year-old patient died on July 14 but her case was referred to detectives yesterday.
The 83-year-old man died today after he suffered a hypoglycaemic episode on July 11.
The deliberate contamination of saline solution with insulin is being investigated. Mr Hopkins said: "As with the deaths of George Keep, Arnold Lancaster and Tracey Arden, the cause of this man and woman's deaths is not known and it is important we do not lose sight of this fact.
"In relation to the death of the woman, this was referred to us by the coroner and after a review of the circumstances surrounding this death - notably the low blood sugar level - we have decided to investigate further.
"In relation to the man's death, due to the fact he suffered a hypoglycaemic episode during a timeframe we are looking at it is only right we conduct further inquiries.
"We have family liaison officers with both families and my thoughts are with them as they are with the relatives of all affected by this incident.
"I want to make it abundantly clear that we are working closely with the coroner and, as is to be expected, it is likely we will be asked to investigate further deaths.
"In the main these are likely to be deaths of people who are elderly and/or ill, and we fully support the coroner's 'belt and braces' approach to ensuring future deaths that require further investigation are appropriately investigated and scrutinised.
"I cannot emphasis enough how complex an investigation this is, requiring detailed forensic and medical analysis, and want to take the opportunity to thank the hospital and staff for their on-going help and support.
"Our inquiry has gathered apace and while we have made an arrest any suggestion that this investigation is close to being complete is misleading.
"All I can say in relation to the arrest is that we have a 27-year-old woman in police custody who has been arrested on suspicion of murder and I am not prepared to say anything else that could potentially prejudice a future trial.
"We are fully committed to finding out exactly what has happened and officers continue to work around the clock in order to provide answers to the families of those who have lost their lives.
"We are determined to identify and bring to justice the person responsible and we are continuing to appeal to anyone who might have information relevant to this investigation to get in touch."
The officer clarified earlier reports suggesting a man in his 40s had died. The officer said he was in fact still very poorly.
A fourth patient linked to allegations of sabotage at a hospital had died earlier.
The man, who was in his 40s, had been "very poorly" for several days.
Stepping Hill Hospital in Stockport confirmed his death today as police continued to question a nurse arrested on suspicion of the murder of three Stepping Hill patients.
Rebecca Leighton, 27, was arrested yesterday at her home in Stockport, just a mile from the hospital where she worked and where the deaths took place.
Tracey Arden, 44, George Keep, 84 and Arnold Lancaster, 71, are thought to have died following the deliberate contamination of saline solution with insulin.
The Nursing and Midwifery Council (NMC) confirmed that Leighton had been arrested.
Her registration will be suspended while she remains under investigation, the organisation said.
Police are expected to give further details about the latest death at a press conference later this afternoon.
A hospital spokesman said: "We can confirm that a man aged in his 40s who was critically ill has died."
Leighton describes herself on her Facebook page as engaged to be married and a "happy go lucky kinda gal"
"Loves the weekend (if im not workin) and having a laugh with the people that I call friends for a reason x", she wrote.
Below her employer details, Leighton states she is a staff nurse, adding: "god it hard work x".
Neighbours said she had lived in the flat with her fiance above a darts equipment shop in Buxton Road for about 12 months.
She was described as a "sociable and pretty young girl".
Police searched the property yesterday afternoon before taking away six bags of items and computer equipment for further examination.
A significant police presence remains at the hospital, where more than 50 staff have been questioned.
Detectives were called in last week after an experienced nurse reported a higher than normal number of patients on her ward with "unexplained" low blood sugar levels, raising suspicions that saline ampoules were being sabotaged.
Officers found insulin had contaminated a batch of 36 saline ampoules in a store room close to ward A1 of the hospital.
A team of 60 Greater Manchester Police detectives are involved in the inquiry with wards A1 and A3 the centre of the complex investigation.
The treatment of 10 other patients, who were affected by the contaminated batch of saline from July 7 but have since recovered, is also being probed.
Medics are now working in pairs to check and administer drugs across the hospital.
Bosses also brought in rules so a single key holder has to be asked for access to locked drugs cabinets and names and times are being taken each time drugs are given to patients.
Police have until early this evening to continue questioning Leighton before they must decide whether to charge her, release her or apply for more time to keep her in custody.
Police stress they are still treating the deaths as unexplained and the cause of death of the patients is as yet unknown.
Police today named an 83-year-old man whose death is being linked to allegations of sabotage at Stepping Hill Hospital in Stockport as Alfred Derek Weaver.