What is the lesson Scrooge learns in Stave 4 that he had not learned before?
In Stave 4, Scrooge learns the truth about the value of his life as it applies to other people. What he comes to see through the lessons of the final spirit, the Ghost of Christmas Yet To Come, is that when the final tally is taken, his life, in the eyes of his fellow man, will be worth nothing. He has not left a lasting legacy; there will be no one to mourn his passing. His life will be reduced to a group of haggling thieves robbing him and stealing his bed clothes, with no respect for his dead body.
"'What do you call this?' said Joe. 'Bed-curtains?' 'Ah.' returned the woman, laughing and leaning forward on her crossed arms. 'Bed-curtains!' 'You don't mean to say you took them down, rings and all, with him lying there?' said Joe. 'Yes I do,' replied the woman. 'Why not?' 'You were born to make your fortune,' said Joe, 'and you'll certainly do it.'" (Dickens)
Scrooge is finally able to see clearly how to measure the life of a man, and it has nothing to do with money. The true value of a man lies in how he has walked among his fellow men, the lives he has touched, made better for having known him. At the moment that Scrooge sees his own headstone, cold, bare and devoid of any sentiment, he is humbled, he is frightened, and he is determined to change.
Also, by this point in the story, he has come to understand the coldness of his own heart, and how punishing it has hurt not only others but also himself. He has rejected all attempts by Fred, his nephew, to be a family. He has mistreated Bob Cratchit, his lowly clerk who is rich compared to Scrooge in terms of love and family.
Mr. Scrooge is transformed in this Stave. He wakes up from his dream to discover that he will be given another chance at life.
"In his agony, he caught the spectral hand. It sought to free itself, but he was strong in his entreaty, and detained it. The Spirit, stronger yet, repulsed him. Holding up his hands in a last prayer to have his fate reversed, he saw an alteration in the Phantom's hood and dress. It shrunk, collapsed, and dwindled down into a bedpost." (Dickens)
"'I will honour Christmas in my heart, and try to keep it all the year. I will live in the Past, the Present, and the Future. The Spirits of all Three shall strive within me." (Dickens)