A 12-year-old boy has been arrested on suspicion of robbery and grievous bodily harm after a 13-year-old was stabbed in the back.
The victim was attacked in an alleyway close to Aldi supermarket in Larkman Lane, Norwich, at about 17:40 GMT on Wednesday.
He was taken to hospital, but police said the wound was not life-threatening and he was discharged later that night.
The 12-year-old was arrested in Norwich on Tuesday morning and has been bailed.
Police said inquiries were continuing and they were still appealing for witnesses and information.
Two other boys, aged 14 and 16, were arrested last week and also remained on police bail.
Police said the victim was approached by one or more suspects, who demanded money before he was then stabbed, although nothing was taken.
Investigators said those involved were thought to be known to one another.