Some customers had inadvertently downloaded software enabling their money to be siphoned off, police said.
During raids at properties in Enfield and Islington on Tuesday, police also removed a live hand grenade.
Two men, aged 31, and two women aged 24 and 27, were arrested on suspicion of conspiracy to defraud and conspiracy to launder money.
They have also been arrested on suspicion of possessing an explosive following searches at an address in Stadium Mews, Islington and Bowes Road in Enfield.
The men are currently in custody while the women have been bailed until early next year.
Officers from the Met's cyber crime unit carried out the operation after being alerted by two UK banks that a number of customers had downloaded complex malware on to their computer by opening emails claiming to be from their bank.
A total of £1m was siphoned off from their bank accounts and transferred to a series of other accounts, to be laundered and withdrawn as cash, police said.
During the searches, computers, smart phones and other media devices were collected to be forensically examined. Police removed and made safe a live hand grenade from one property.
Luxury goods including designer jewellery and a Range Rover were also seized.
Officers have served restraint requests to several banks for a number of accounts linked to the investigation to be frozen.
Det Ch Insp Jason Tunn said: "The victims have been hoodwinked by malware-carrying emails purporting to be from their banks, and subsequently had money taken from their accounts."