In Chapter 4, why does Roger, throwing stones at one of the littluns, aim to miss?
Roger is a bully. He entertains himself by producing reactions of sorrow, anger, or fear in others because that makes him feel powerful. As Roger and Maurice come down to the beach, Roger heads right for the littluns who are playing there, destroying their sandcastles with their landscaping. This actually doesn't produce the result Roger was hoping for because the children are not at that moment concerned with the castles Roger has destroyed. Only Percival starts crying because of the sand Maurice, and then Johnny, gets in his eyes. So Roger continues to watch the littluns, perhaps looking for another way to get under their skin. When Henry wanders off down the beach, Roger follows him surreptitiously, certainly with mischief in mind.
Interestingly, Henry stops to entertain himself with some little transparent creatures living in the tide pools. In a way, Henry is doing to the sea creatures what Roger seeks to do to him. Henry blocks the path of the sea creatures and confuses them without actually bringing any physical harm to them. Yet "he became absorbed beyond mere happiness as he felt himself exercising control over living things."
Roger, hiding from Henry under some palms, is showered by falling nuts that don't actually hit him. This gives him the idea to confuse and tease Henry. He picks up some stones and throws them around Henry, taking care not to hit him, not because of any concern for Henry but merely in observance of a "taboo of the old life." Henry looks around each time a rock falls near him, and Roger hides. In this way, Roger creates an emotion of confusion in Henry. Again, however, it is not the exact emotion Roger seeks. Roger was trying to create fear, but Henry laughs off the teasing and walks away. Nevertheless, Roger is satisfying his need for power and control, to make someone else yield to him. Later this innate desire for control and mastery finds fulfillment in his delivering the death stab to the sow and in his rolling the rock that kills Piggy.
Roger's actions are at the center of Golding's theme of the inherent savagery in man. After Roger and Maurice emerge from the forest, Roger exhibits his sadism kicks over the sand castles the litt'uns have built. When Percival whimpers with sand in his eye, Maurice recalls having received "chastisement for filling a young eye with sand," so he becomes uncomfortable and mutters that he is going for a swim and leaves Roger to himself.
He was not noticeably darker than when he had dropped in, but the shock of black hair, down his nape and low on his forehead, seemed to suit his gloomy face and made what had seemed at first an unsociable remoteness into something forbidding.
Roger follows Henry, hiding behind the palms. Then, he waits. Roger bends down, picks up a stone, "that token of preposterous time" and bounces a stone a few yards away from Henry.
Here, invisible yet strong, was the taboo of the old life....Roger's arm was conditioned by a civilization that knew nothing of him and was in ruins.
Indeed, it is Roger's conditioning in society which prevents him from striking Henry; there is yet some fear of punishment for a cruel act. But, as time goes on and the boys have no adult to supervise them, Roger begins to release his sadistic yearnings.
The author says that Roger imagines about a 6 foot radius around the littlun that he is throwing rocks at - that radius is created by Roger's memory of parents, teachers, schools, and basically anyone in authority from the boys' former lives. The text says that "Roger's arm was conditioned by a civilization that knew nothing of him and was in ruins."
This is another of Golding's examples of the breakdown from civil to savage. As you've referenced, Roger will not hut the littluns with small stones early in the novel, yet by the end, he has no problems launching a large boulder on top of Piggy.