The Collaborative International Dictionary of English v.0.48:

All fours \All` fours"\ [formerly, All` four".]
   All four legs of a quadruped; or the two legs and two arms of
   a person.
   [1913 Webster]

   To be, go, or run, on all fours (Fig.), to be on the
      same footing; to correspond (with) exactly; to be alike in
      all the circumstances to be considered. "This example is
      on all fours with the other." "No simile can go on all
      fours." --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

on all fours
    adv 1: on hands and knees; "he got down on all fours to play
           with his grandson"