1
general::
On one's hands and knees, as in Seven of us were on all fours, looking for the lost earring in the sand. In this idiom fours refers to the four limbs. [1300s]
American Heritage Idioms
2
general::
Phrase(s): on all fours
on one’s hands and knees. • I dropped a contact lens and spent an hour on all fours looking for it. • The baby can walk, but is on all fours most of the time anyway.
McGrawhill's American Idioms And Phrasal Verbs