A 16-year-old boy has been arrested in Norwich in connection with alleged data theft from TalkTalk.
The teenager is being held on suspicion of Computer Misuse Act offences.
He was detained after detectives from the Metropolitan Police Cyber Crime Unit (MPCCU) and officers from the National Crime Agency (NCA) executed a search warrant.
The youngster is the fourth person to be arrested in connection with the cyber attack on the telecoms firm.
A 20-year-old man, detained at an address in Staffordshire on Saturday, has been bailed to attend a police station in early March pending further inquiries.
Two teenagers have previously been arrested over the incident.
Another 16-year-old boy, from Feltham in west London, was taken into custody after a search of his home on Thursday. He was later bailed to a date yet to be confirmed.
A 15-year-old boy, from County Antrim in Northern Ireland, was arrested on Monday and released on police bail until a date in November.
They were all arrested on suspicion of committing offences under the Computer Misuse Act.
The latest breach is the third in a spate of cyber incidents affecting TalkTalk in the last year.
The company said it had been subjected to a "significant and sustained" attack on its website on October 21, which prompted fears that millions of people may have had their bank details stolen.
The telecoms giant has since said the data hacked was "significantly less than originally suspected" with fewer than 21,000 unique bank account numbers and sort codes accessed.