Of course this is a matter of opinion, but the phrase "four great plays" usually refers to the four major tragedies: "Hamlet" (around 1602), "Othello" (1604), "King Lear" (1605) and "Macbeth" (1606.) (If you think of "HOLM", they're quite easy to remember.) They are often bracketed together, not only because many people do regard them as Shakespeare's finest writing, but also because he wrote them in such close sequence and they have so many points of comparison.
Perhaps the most important similarity uniting these plays is the way they concentrate, to a degree rarely seen in drama before that time, on the development of a single, central character. In each play, the protagonist (main character) changes profoundly in the course of the action, and the audience is involved in the change as we follow the character's private thoughts. When Macbeth, for instance, changes from a loyal soldier to a bloodthirsty tyrant, we understand how this has happened; this ability to create empathy and involvement with characters, no matter what they do, is what makes the four plays "great" in the eyes of many.
Perhaps the most important similarity uniting these plays is the way they concentrate, to a degree rarely seen in drama before that time, on the development of a single, central character. In each play, the protagonist (main character) changes profoundly in the course of the action, and the audience is involved in the change as we follow the character's private thoughts. When Macbeth, for instance, changes from a loyal soldier to a bloodthirsty tyrant, we understand how this has happened; this ability to create empathy and involvement with characters, no matter what they do, is what makes the four plays "great" in the eyes of many.
