A second teenager has been arrested on suspicion of murder following a stabbing outside Parsons Green Tube station.
Omid Saidy, 20, was killed outside the London station on Monday, where 30 people were injured in a terror attack last month.
He was knifed after asking a drug dealer and another man to leave the area, Metropolitan Police said.
Scotland Yard said an 18-year-old man has been arrested at an address in Mitcham, south London, on suspicion of murder.
Two females and another male were also arrested at the address on suspicion of assisting an offender, the Met said, with all four in custody at a central London police station.
On Thursday a 16-year-old boy, who cannot be identified, appeared in custody at Wimbledon Youth Court, charged with murder and threatening another man, Oluwafemi Omotosho, with a knife.
The teenager, from Shildon, County Durham, was also injured during the attack and was arrested after being released from hospital.
He appeared in the dock wearing a grey tracksuit and spoke only to confirm his details during the short hearing.
He was remanded in custody and ordered to appear at the Old Bailey on October 23.
The boy’s stepfather sat in the back of the court.
Mr Saidy, from Fulham, was pronounced dead in Parsons Green Lane at 8.30pm.
Police believe the incident started at around 7.30pm by a Co-Op supermarket on Parsons Green Lane before moving down the road and past the station.
An 18-year-old man, a friend of the victim who arrived on a moped, was also stabbed in the attack and is still in hospital with serious injuries, police said.
He has been arrested on suspicion of affray.