What are four symbols in The Great Gatsby by F. Scott Fitzgerald? Why are they significant, and what do they teach us about society and the...

The green light at the end of Daisy's dock is symbolic, in many ways, of the American Dream.  It represents hope for the future, that if a person works hard enough and long enough, they can prosper and be happy.  Nick sees Gatsby reaching out toward the light at the end of the first chapter, and in the final lines of the novel, he calls the light "the orgastic future that year by year recedes before us.  It eluded us then, but that's no matter -- to-morrow we will run faster, stretch out our arms farther. . . . And one fine morning--."  We can, put simply, never actually reach the light.  No matter how hard we work or try or dream, we can never really reach that American Dream, and its elusiveness ought to make us pity Gatsby, and even Myrtle and George Wilson, as victims of its unattainability.

In Chapter IV, having been reunited with Daisy after five years, Gatsby almost breaks a clock at Nick's house when Nick invites them both for tea at Gatsby's request.  Gatsby longs, and believes that it is possible, to repeat the past.  He tells Nick this very thing later in the story.  However, it is not possible to stop time or to revisit the past, and the fact that he almost breaks the clock represents his desire to stop time, while the fact that he doesn't break the clock symbolizes his inability to do so.

The valley of ashes, where the Wilsons live, is a place where everything is so covered in ashes that the houses and people and landscape appear to be made of ashes.  It is the product of industry, the industry that makes just a few very, very wealthy and leaves the majority with so very little.  Wilson, for example, works so hard, but he cannot earn enough money even to be able to afford to move elsewhere.  The valley, therefore, symbolizes the great gulf between those few individuals who prosper and the vast majority who are used up, reduced to ash, by the demands of the prosperous.  It certainly helps us to understand and sympathize with someone like George Wilson.

The deaths of Gatsby and the Wilsons is symbolic of the death of the American Dream.  When Gatsby tries to reach it honestly, he finds that it is impossible, and so he attempts to reach it illegally (which means that he does not actually achieve it).  Then, George and Myrtle each try to better their situations, in different ways, and neither is successful either.  No one can really get what they want in a world that is corrupt enough to give all the privilege to terrible people like Tom and Daisy simply because they possess "old money" and status.