What role do the Golden One and International 4-8818 serve in the novella Anthem by Ayn Rand? How do they contribute to our understanding of Equality 7-2521 and the society around him?

These two function as the human support for Equality 7-2521. Our understanding of the overall society in Anthem is enhanced by these two because they are a contrast to it and whom Equality 7-2521 can bond with in a way he cannot with other people. Rand's main point, in Anthem and in all her books, is that all achievement is the work of men alone, individuals who follow their own path—not out of duty to others but to themselves. But such people can't exist in a vacuum. They need friends (this is where International 4-8818 comes in) and a woman—in this case Liberty 5-3000, the Golden One. These two are people who think similarly to Equality 7-2521 and, thus, become attached to him in his quest to re-discover the forbidden knowledge of the past.
Anthem has many elements in common with other dystopian works. The Golden One functions much as Julia does in 1984. Equality 7-2521 is similar to Winston, a man defying the regime and the inhuman society he is trapped in. The hero in the dystopia genre yearns for a vanished past that existed before a cataclysm of some kind destroyed it and replaced it with the nightmare world of the present. Such is also John, the "Savage" in Brave New World. What distinguishes Rand from Huxley and Orwell is her simplistic, fable-like approach (her hero singlehandedly re-invents electricity just like that) and her subtext of a "philosophy" she would later call "Objectivism," based on the notion that altruism, which she defines as putting the good of the many above that of the individual and thereby squelching the individual, is evil. In each of her novels there is at the center a kind of "power couple," like the two in Anthem, Equality 7 and the Golden One, who are a man of almost superhuman intellect and a woman whose intelligence and character matches his. Such are Roark and Dominique in The Fountainhead and Reardon and Dagny in Atlas Shrugged. There is usually a third man who almost equals the superhero, but is flawed in some way, and is in love with the same woman. International 4-8818 is a kind of undeveloped version of this third-wheel character.
Apart from these incorporations of her standard characters and themes, Anthem differs from other dystopian fiction in one further respect: the story ends happily. In her critical writings, Rand stated that she despised literature that showed man defeated—though her favorite novel was Victor Hugo's Ninety-three, about the French Revolution, at the end of which the hero Gauvain is guillotined.