This is the scene where Roger is throwing stones at Henry, one of the littleuns. It is significant because this is where we start to notice the boys losing their civilian nature. The boys are starting to digress into animal-like behavior, with the big taunting the small, or the strong overpowering the weak. In a way, it's the beginning of the end of their civilized ways. Even when he's throwing the rocks, Roger essentially remembers something about it being wrong; however, that voice was distant, in the outer realms of his memory, so he throws the rocks at Henry anyway.
In chapter 4, the boys' society is still functioning fairly normally. Roger has just taken his turn at tending the signal fire, doing his part for the society, and he heads to the beach for his free time. As soon as he gets there, he plows through the sandcastles the littluns, including Henry, have made. Roger appears to be a natural bully; this is the type of behavior that occurs the world over. During this scene, Maurice has accompanied and mimicked Roger, but when one of the boys gets sand in his eyes, Maurice feels guilty and moves on. Roger, however, is a person who already has an "unsociable remoteness," suggesting that he cannot as easily put himself in other people's shoes. Therefore, he doesn't really think about causing pain or hurting the feelings of others; he only considers how it makes him feel. In this case, he was actually hoping to create a stir among the littluns, probably to make some excitement and to feel the power of controlling others. The castles the older boys destroyed were no longer central to their game, so the littluns don't really fuss much about their destruction. Roger doesn't get the reaction he is looking for.
Henry wanders off along the beach and begins playing with little sea creatures in the tide pools, capturing them in tiny lakes formed by footprints in the sand. Roger follows him unseen; he hides behind a tree and from there begins throwing small stones at Henry, throwing to miss. He lets the rocks fall in a circle around Henry, never coming closer than three feet to him. Henry looks around to see who is teasing him, but Roger won't reveal himself. This passage shows that Roger is a bullying type and that he gains satisfaction from producing unpleasant emotions in others. The action of throwing the rocks was not particularly harmful; a friend might have done it at as a joke and then revealed himself to Henry, and it could have been a bonding experience. However, Roger's desire to control others without creating relationships becomes a danger to the boys later on when Roger teams with Jack and with him executes "irresponsible authority" over others.
Roger deliberately throws rocks AROUND Henry, not to hurt him but simply to instill fear in him for Roger's pleasure. Roger's hold on civilization is obviously slipping, even more so than the other boys at that point.