between you me and the bedpost
between you (and) me and the bedpostand between you and me and these four walls
Fig. a somewhat affected way of signaling that you are about to tell a secret. Alan: What's wrong with Ellen these days? She seems so touchy. Jane: Between you and me and the bedpost, I've heard that her boyfriend is seeing someone else. Jill: How much did you get for your used car? Jane: Well—between you and me and these four walls—five thousand dollars.