Scene iv is very short. Honey returns to tell the others that Martha is changing so as to be more comfortable. Honey wants to know more about George and Martha's twenty-one-year-old son, and George is stunned to hear this news and threatens to get even with Martha in some way.
While this scene is very short, it is also a turning point in the drama. George is aware, first, that if Martha is changing clothes, she is changing for the hunt, for the kill, the seduction. As George tells the young couple, "Martha hasn't changed for me in years" meaning not that she has not changed her clothes, but her basic lifestyle. Only at the end of the drama will Martha have to face the fact that she must change — she can no longer live as she has.
When George hears that Martha has mentioned their "son," he wheels around "as if struck from behind." His insistence upon hearing it again as though he were "nailing it down" (stage directions) indicates clearly that this is the first time that the son has been mentioned in front of others. His first reaction is an ominous warning with threatening overtones. The fact that an outsider now knows about this means that George cannot allow them to leave with the knowledge of their imaginary son — that he must keep the guests in his house until he can discover what to do.