How does the author help us to imagine the feelings of the people in the camp in chapter 4?

In chapter 4, Bruno and Gretel watch the prisoners in the camp. They notice that some of the prisoners stand "near the huts in quiet groups, staring at the ground as if it [is] the sort of game where they [don't] want to be spotted." There is a bit of dramatic irony here, in that the children don't quite understand what is happening. Indeed, they don't understand that these people are prisoners.
This perhaps heightens our sympathy for the prisoners because we, of course, realize that they are not players in a game at all. The fact that they all keep their heads down also helps us to understand how disconsolate and afraid they must feel. They are afraid that if they make themselves noticed for the wrong reasons, then they will be punished.
A little later in the chapter, Bruno points to a group of children who are being shouted at by a group of soldiers. Bruno and Gretel notice that "the more they [are] shouted at, the closer they huddle together." This image conveys how scared the children are. They instinctively huddle closer together because they feel that they need protection. Gretel sees that some of the children "look as if they [are] crying," which clearly points to their sadness and fear.
At the end of the chapter, Bruno thinks about the fact that all of the people at the camp are "wearing the same clothes as each other: a pair of grey striped pajamas with a grey striped cap on their heads." We can infer from this that the prisoners must feel somewhat dehumanized: they are not treated like individuals, but rather as one, homogeneous group.