The Collaborative International Dictionary of English v.0.48:

4 \4\ adj.
   1. one more than three; denoting a quantity consisting of
      four items or units; -- representing the number four as an
      Arabic numeral.

   Syn: four, iv
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

4
    adj 1: being one more than three [syn: four, 4, iv]
    n 1: the cardinal number that is the sum of three and one [syn:
         four, 4, IV, tetrad, quatern, quaternion,
         quaternary, quaternity, quartet, quadruplet,
         foursome, Little Joe]