A 16-year-old boy arrested in Norwich in connection with an alleged data theft from TalkTalk has been released on bail until March.
He was held on suspicion of Computer Misuse Act offences after officers searched a property in the city.
Police have also arrested and bailed a 16-year-old from London, a 15-year-old boy in County Antrim, Northern Ireland, and a man, 20, from Staffordshire.
The TalkTalk website was hit by a "significant" hacking attack last week. On Friday, the phone and broadband provider confirmed that at most 1.2 million email addresses, names and phone numbers and 21,000 unique bank account numbers and sort codes were accessed in the attack.
It was fewer than originally thought, the firm said.
What should you do if you think you're at risk?
Report any unusual activity on your accounts to your bank and, if you are in England, Wales or Northern Ireland, to the national fraud and internet crime reporting centre Action Fraud on 0300 123 2040 or www.actionfraud.police.uk. If you are in Scotland, call Police Scotland TalkTalk is advising customers to change their account password as soon as its website is back up and running and any other accounts for which you use the same password.
Beware of scams: TalkTalk will not call or email customers asking for bank details or for you to download software to your computer, or send emails asking for you to provide your password
What should I do?
The company, which has more than four million UK customers, said it would be writing to all affected people to let them know what information had been accessed.
It said that any stolen credit or debit card details were incomplete - and therefore could not be used for financial transactions - but advised customers to remain vigilant against fraud.
The fourth arrest in connection with the attack came after detectives from the Metropolitan Police Cyber Crime Unity and the National Crime Agency executed a search warrant at the Norwich address, the Met said.