A 54-year-old man attacked in a Norwich car park died of head injuries, a post-mortem examination has found.
The man, who was Portuguese and lived in the city, was attacked on 10 February and died four days later in hospital.
Police are trying to contact his family in Portugal before they identify him.
Two men aged 19 and 21 and two women aged 33 and 49, who were arrested on suspicion of grievous bodily harm with intent, remain on police bail.
Officers from Norfolk and Suffolk's joint major investigation team (MIT) said the murder inquiry was at an early stage and appealed for witnesses to come forward.
Det Ch Insp Neil Luckett said: "This investigation is still in its early stages and identifying the motive for the attack is a key line of inquiry.
"At this time I retain an open mind about why this attack took place.
"I would ask people who were in the Rose Lane area of Norwich between 2200 GMT on 9 February and 0200 GMT on 10 February to contact my team urgently with any details they feel could be significant to our investigation, in particular if they were in the Rose Lane car park or the alleyway next to it."
The victim's partner, 42, suffered minor injuries in the attack.