Definition: The literal walls of a house in this dystopian society are replaced by installing big television screens. Specifically the fourth wall or the last wall within Montag's home that isn't a giant television screen
Firemen
Definition: To the public eye, these firemen are the heroes saving the world from evil blasphemous books, although their whole job is to burn books for the purpose of taking away people's ideas, identifies, and thoughts.
Mechanical Hound
Definition: Mechanical Hounds are "Dogs" made of metal that is used by the fire department to hunt down people who are suspected to be in possession of banned books and instill fear and intimidation the people into submission.
The significance of the fourth wall is how it represents the rift between Mildred and Guy Montag, as she wants to buy the fourth wall to be surrounded by the screens and become entranced by this fake reality. She is so caught up in her own world of watching and listening to the television screens that she didn't realize or want to realize the reality that Guy was actively pursuing, thus causing a disconnect between Guy and Mildred and ultimately caused their marriage and relationship to get worse and fail over time.
The sounds of the screens that were the walls, encapsulated the room, only one wall was clear of screen, the fourth wall.
The significance of firemen is that Guy Montag is one of them and because of their close involvement with book-burning it gives Guy the perfect access to some books making him become curious about what is written inside them and what he really perceives as truth.
The people revered them as heroes as the firemen burned the books in front of them.
These hounds highlight the government's insane usage of technology to control and manipulate. Sequentially, Guy Montag is particularly saddened by his interactions with the hounds because he desires for them to be friendly highlighting how he wishes the society he lives in was different and instead warmer, non-robotic, authentic, and social.
The mechanical hound hunted down with great threat and intimidation he woman reading the books as she was violating the law.