Officers were called to the incident in Whitehawk, Brighton, at 2.30pm today (April 5).
A man, believed to be in his 20s, was taken to Royal Sussex County Hospital in Brighton with serious injuries, where he sadly died, police have confirmed.
Four men were subsequently arrested on suspicion of murder, the force said. All four suspects remain in police custody at this stage.
Detective Chief Inspector Alex Geldart, of the Surrey and Sussex Major Crime Team, said: “Our thoughts are with the victim and his family at this very difficult time.
“This is believed to be an isolated incident and we have four suspects in custody; we are not seeking anyone else.
“This is a fast-moving investigation and I’d like to urge people to refrain from speculating on social media.
“If you saw what happened, or if have any information which you think could assist in our investigation, please contact us.
“Local residents can expect to see a significant police presence in the area as our enquiries continue, and I’d like to thank everyone for their patience and understanding.”
Anyone with any information about the incident is urged to report it online or call 101, quoting operation Heathland.
People can also report it anonymously to Crimestoppers on 0800 555 111 or online.