WordWeb Online

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

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

See also: multiple