why is stave 4 nessary when scrooge has already changed so much?

Dickens continues his development of the theme of free will over determinism. Scrooge understands that the future he is shown is alterable and that he can change his fate. Again, this idea celebrates the potential for redemption in anyone and urges people to change their ill ways right now as opposed to later. It remains to be seen if Scrooge will follow through with his new attitude towards life.