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