on all fours

from 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"

from 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]