Superstorm Sandy knocked off nearly $4 billion from retail sales last week in the hard-hit Mid-Atlantic and Northeast regions, nearly 20 percent of the usual total, according to a retail data service.

MasterCard Advisors’ SpendingPulse says that the regions, which account for about 24 percent of retail sales nationwide, typically generate $18.7 billion in sales for the week ended Saturday. But revenue came in at about $15 billion. The figures exclude auto sales.