The police watchdog is investigating whether a man found dead on a village road was assaulted in police custody.
Gareth Roper, 35, was found in Lily Lane in the Bamfurlong area of Wigan in the early hours of New Year's Day.
Officers attended at about 4.30am and Mr Roper was pronounced dead at the scene.
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Greater Manchester Police said it had referred itself to the Independent Office for Police Conduct (IOPC) and that the force was "wholly committed" to supporting the investigation.
The IOPC said it would "look at the actions and decision-making of officers".
A spokesperson for the IOPC said: “We are currently independently investigating four Greater Manchester Police officers who had contact with Gareth Roper in the hours before his death in Bamfurlong, Wigan, on 1 January.
“This will look at the actions and decision-making of officers when Mr Roper was arrested and subsequently de-arrested, including any considerations for his welfare.
"We are also looking into an allegation that Mr Roper was assaulted while in custody.
“We will be reviewing evidence including police body-worn video and CCTV footage, as well as radio transmissions, as part of our inquiries.
“Our thoughts remain with Mr Roper’s loved ones and all those affected by his death.”
A GMP spokesperson said: "Due to previous police contact with Mr Roper prior to his death, Greater Manchester Police referred this case to the Independent Office for Police Conduct who are now leading the investigation into this case.
"This is clearly a very tragic incident whereby a family has lost a loved one, and we are wholly committed to supporting the IOPC with their investigation to ensure the circumstances prior to Mr Roper's death are fully understood, and should any wrongdoing be found, appropriate action will be taken."
