What is the link which has snapped between Ralph and Jack at the end of Chapter 4 of "Lord of the Flies"?
It's a difficult question precisely to answer: the link that has snapped is one of loyalty, trust, friendship and of shared values. Remember that Golding specifies that “not even Ralph knew how a link between him and Jack had been snapped and fastened elsewhere.”
That elsewhere is Piggy, who shares Ralph's concern for the fire: and, for the first time in the novel, Chapter 4 shows Jack disregarding Ralph's obsession with the fire in favour of hunting (a gulf that widens as the novel progresses). Ralph tells Jack about the ship: Jack is more interested in meat. Golding is showing us the two boys' ideologies forming in different directions:
The two boys faced each other. There was the brilliant world of hunting, tactics, fierce exhilaration, skill; and there was the world of longing and baffled common-sense.
Ralph doesn't quite have the strength of his convictions, though: note the way he nearly laughs when Jack punches Piggy, and how Jack's hunting lifestyle remains attractive to Ralph throughout.
Yet what Ralph knows, as this link is refastened, is that Piggy - and the clear-sightedness his glasses, and the fire, represent - is a stronger ideological ally for him than Jack: and this knowledge is strengthened by the chapters which follow.
Towards the end of Chapter 4, the burdens of responsibility vs. popularity begin to drive Ralph and Jack further apart. Ralph, Piggy and Simon are trying to get the boys to do what is necessary and Jack allows the boys to do what they want. Jack sees this as a way of becoming more popular. He may not be able to keep a fire going in order to get rescued, but he can kill a pig and roast it in a fire to give the boys the immediate gratification of eating meat. Thus, the choir and other boys begin to gravitate to Jack because he gives them what they supposedly want while all Ralph does is harp at the boys for not doing what they need to do.