This line cannot be fully appreciated without picturing what is happening on the stage. Lear enters carrying the dead body of Cordelia in his arms. Everyone is frozen and struck dumb by the sight. When Lear says, "Howl, howl, howl, howl!" he is not addressing the group as a whole but addressing the word "howl" to four separate individuals. They do not move and do not respond because they are frozen, rather surrealistically, in whatever positions they were in when Lear entered. Then when Lear tells these four individuals, "O, you are men of stones," he is describing literally what the audience sees. Not only are the men as motionless as stone statues, but they appear to have no more feeling than statues. Lear says they are "men of stones," not "men of stone," because he wishes to convey the idea that each has been carved out of a separate stone. If Lear entered and simply cried out, "Howl, howl, howl, howl!" it would not only be ineffective but awkward. An actor would find it hard to say the words naturally. But if Lear looks at one person with each utterance of the word "Howl" and if each person fails to react, the four imperative "howls" can be effective. Lear wants somebody to howl, and he can't get anybody to do it. He thinks this is because they don't care about his daughter or his grief, but the opposite is true: they are paralyzed with horror at the sight. They don't know what to say or do. The audience is horrified at the sight, too. They were hoping there might be a chance that Cordelia could be saved at the last moment.