The family is vitally important here, and is capable of having an overwhelming effect. Here, Larkin's famous 'They fuck you up, your mum and dad' can be extended to the whole family. But, apart from the unnamed mother, the only original survivors are the older brother and sister Saul and Hélène, and the younger twins Élias and the Réna. Initially the story seems to be mainly about the mother selling the original family home after the death of the father, the house now being too big for the mother, and anyway she wants to free money up for herself. And then there are the undecided details of how the remaining money should be parceled out after her death: is it fair to allow each brother or sister to have an equal share, when Saul and Hélène clearly have much more money than the two younger siblings, of whom Réna in permanently on crutches due to a car accident.
Yes, it's more than about the house, it's the four human walls too, it's about sibling rivalry in this particular family, about previous events in family history, and about incest, although that (in its physical expression at least) only involves Hélène and Dimitri (now dead in a car crash, although we learn (eventually) that he wasn't driving).
With Quatre murs we're in the realm of the often unspoken, but where even the slightest word can be construed as criticism, indeed attack. It's all pretty stormy.