Chapter four of William Golding's Lord of the Flies is significant in many ways, but it is the reference to "painted faces" in the title which is probably the most significant. 

In this chapter, we learn that some time has passed (this the reference to "long hair" in the title), but the boys are still civilized enough to refrain from breaking the accepted rules of society. When Roger throws rocks at Henry, he avoids throwing right at the boy.

[T]here was a space round Henry, perhaps six yards in diameter, into which he dare not throw. Here, invisible yet strong, was the taboo of the old life. Round the squatting child was the protection of parents and school and policemen and the law. Roger’s arm was conditioned by a civilization that knew nothing of him and was in ruins.

Though the rest of the world is in chaos, even one of the most aggressive boys on the island feels the restraints of civilization on his behavior.

Another significant moment in this chapter is when everyone learns that Jack and his hunters have let the fire go out; even worse, a ship passed by without stopping because there was no smoke to signal it. This incident displays a shift in Ralph which also indicates a shift in power. While Jack and Ralph are not really friends, they have generally, until now, managed to work together. Now, Ralph finds himself in a standoff with Jack which results in a new relationship between Ralph and Piggy.

Not even Ralph knew how a link between him and Jack had been snapped and fastened elsewhere.

This will be a vital relationship as the novel progresses.

The biggest thematic moment in the chapter, however, occurs when Jack paints his face. He has been unsuccessful at hunting and realizes the pigs have seen him more than smelled him, so he decides to makes some face paint. After he paints his face red and black, he looks at himself in some water.

He looked in astonishment, no longer at himself but at an awesome stranger. He spilt the water and leapt to his feet, laughing excitedly. Beside the pool his sinewy body held up a mask that drew their eyes and appalled them. He began to dance and his laughter became a bloodthirsty snarling. He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness.

Now Jack has a mask behind which he can hide, and this mask frees him from feeling any pangs of conscience or shame.

The natural result of this, of course, is Jack's ability to commit any act he wishes without any restraints or restrictions of civilized society. While we were reminded at the beginning of the chapter that some things are still taboo, Jack has changed everything by the simple application of a painted mask. It is the beginning of Jack's, and soon the entire group of boys', descent into savagery.