WordWeb Online

1. By a factor of four
   "the price of petrol has increased fourfold over the past two years"
   - four times

1. Four times as great or many
   "a fourfold increase in the dosage"
   - quadruple, four-fold
    
2. Having four units or components
   "fourfold rhythm has four beats per measure"
   - quadruple, quadruplicate, quadruplex, four-fold

See also: multiple