A teenage "hacktivist" from Chester is due to be sentenced at Southwark Crown Court for his part in a plot to carry out cyber attacks with the notorious Anonymous group.
Jake Birchall, 18, admitted conspiring to impair the operation of computers between August 1 2010, and January 22 2011.
The group's sabotage cost one website £3.5 million. Birchall's three co-accused were jailed last week.
Christopher Weatherhead, 22, of Holly Road, Northampton, was given an 18-month sentence after being found guilty of the same charge.
Ashley Rhodes, 28, of Bolton Crescent, Camberwell, south London, was jailed for seven months, and Peter Gibson, 24, from Castletown Road, Hartlepool, who was deemed to have played a lesser role in the conspiracy, was given a six-month suspended sentence.
The hacking hit financial sites including PayPal, MasterCard and Visa, which were chosen by Anonymous as part of so-called Operation Payback because the hackers did not agree with their views.
The distributed denial of service, or DDoS, attacks paralysed computer systems by flooding them with a huge number of online requests.
Victims' websites would be directed to a page displaying the message: "You've tried to bite the Anonymous hand. You angered the hive and now you are being stung."
The attacks cost PayPal £3.5 million in loss of trading, paying for extra staff to deal with the aftermath of the attack, and new software and hardware to defend against future attacks.
Anonymous initially targeted companies involved in anti-piracy and digital rights, including the British Recorded Music Industry (BPI), the International Federation of the Phonographic Industry (IFPI) and four sites operated by the Ministry of Sound. They then shifted their attention to PayPal, MasterCard and Visa, which would not process donations to the Wau Holland Foundation, which is involved in raising funds for WikiLeaks.