on all fours
  四肢著地,匍匐著
  All fours  All four legs of a quadruped; or the two legs and two arms of a person.
  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.
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  on all fours
       adv : on hands and knees; "he got down on all fours to play with
             his grandson"