A teenage girl challenged a group of robbers as they attacked a 17-year-old boy in Bury St Edmunds.
The boy was targeted by the group, in St Olaves Road, on Wednesday, March 29 at about 1.20pm to 1.30pm.
They stole his mobile phone before the teenage girl intervened and challenged the attackers who then ran away.
Four teenage boys - a 14-year-old, two 15-year-olds and a 16-year-old, all from Bury St Edmunds - were arrested on suspicion of robbery.
All have been bailed to return to police on June 14 or 15 pending further enquiries.
Detectives investigating the incident are appealing for witnesses, who should call west Suffolk CID on 101.
The crime reference is 25817/17.