Police responded to a report of a fight involving around 20 people in Montpelier Place at 10.15pm on Wednesday (4 November).
A 40-year-old local man suffered stab injuries and was taken to Royal Sussex County Hospital for treatment, where he remains in a serious but stable condition.
Two men aged 18, and two other men aged 19 and 57, were arrested on suspicion of attempted murder. They are assisting police and have each been released under investigation, pending further enquiries.
Detective Inspector James Ansell, of the Surrey and Sussex Police Major Crime Team, said: “This incident happened in a busy area of Brighton and we’re appealing for anyone with any information – including any relevant CCTV, dashcam or mobile phone footage – to contact us.
“We are confident that someone knows what happened and would urge them to do the right thing and speak to us immediately. You can also contact Crimestoppers in confidence.
“The victim and suspects are believed to be known to each other and this is being treated as an isolated incident with no threat to the wider public.”
You can report information either online or by calling 101, quoting Operation Moorcroft.
You can also contact the independent charity Crimestoppers anonymously on 0800 555 111 or report it online.