The Collaborative International Dictionary of English v.0.48:

Four \Four\, n.
   1. The sum of four units; four units or objects.
      [1913 Webster]

   2. A symbol representing four units, as 4 or iv.
      [1913 Webster]

   3. Four things of the same kind, esp. four horses; as, a
      chariot and four.
      [1913 Webster]

   All fours. See All fours, in the Vocabulary. Fourb

The Collaborative International Dictionary of English v.0.48:

Four \Four\ (f[=o]r), a. [OE. four, fower, feower, AS.
   fe['o]wer; akin to OS. fiwar, D. & G. vier, OHG. fior, Icel.
   fj[=o]rir, Sw. fyra, Dan. fire, Goth. fidw[=o]r, Russ.
   chetuire, chetvero, W. pedwar, L. quatuor, Gr. te`ttares,
   te`ssares, pi`syres, Skr. catur. [root]302. Cf. Farthing,
   Firkin, Forty, Cater four, Quater-cousin, Quatuor,
   Quire of paper, tetrarch.]
   One more than three; twice two.
   [1913 Webster]

WordNet (r) 3.0 (2006):

four
    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]
    2: a playing card or domino or die whose upward face shows four
       pips [syn: four-spot, four]