going on all fours

“What is Jack doing there? I don’t know. He is going on all fours.”
What is the meaning for the phrase “going on all fours”?
This is the first time I am hearing.

on all fours - on hands and knees;crawling “he got down on all fours to play with his grandson”

With the given context, I don’t think it literally means “He is crawling.”, but implies that Jack is doing the thing or moving very slowly.

You may be right. James but I take it as a dialogue:“What is Jack doing there?”
-“I don’t know. He is going on all fours” /doing smth strange and extraodinary things never done before/.
2) a person is peeping in and seeing Jack crawling. Then follows his reply":What…" and so on.
Will you come up with examples proving you were right?

I agree with James. It seems to me that the person is likening Jack’s slow, cautious approach with someone who is crawling, though Jack is not necessarily literally crawling. It is a metaphor for moving, acting or speaking slowly and cautiously.