Wiktionary
a. 1 (context idiomatic English) On one's hands and knees. 2 (context idiomatic often followed by "with" English) similar in nature or effect to something else; consistent. adv. 1 (context idiomatic English) On one's hands and knees. 2 (context idiomatic often followed by "with" English) In a manner which is similar in nature or effect to something else; consistently. alt. 1 (context idiomatic English) On one's hands and knees. 2 (context idiomatic often followed by "with" English) similar in nature or effect to something else; consistent.
WordNet
adv. on hands and knees; "he got down on all fours to play with his grandson"
Usage examples of "on all fours".
After I left work we made love on all fours on the living room rug, then again standing in the shower, then on top of the frigging kitchen counter, her legs wrapped around my waist, while waiting for the microwave to ping.
He dared to rise up on all fours, but felt himself shift alarmingly--the bouncing and rocking of the train threatened to dislodge him.