From Longman Dictionary of Contemporary Englishthese four wallsthese four wallsspokenPRIVATE/PERSONAL the room that you are in, especially considered as a private place I don’t want anything repeated outside these four walls. → wall
Examples from the Corpus
these four walls• What purpose did she serve sitting looking at these four walls?• I can not be said to have a life outside these four walls.• I don't want anything that I have said repeated outside these four walls. 