What is chapter 4 about?

Chapter Four details yet another robbery by the talented Flambeau. Sadly, his thievery places innocent, young man's life at the mercy of the police, and Father Brown is having none of it. He convinces Flambeau to give up the jewels, after which, Flambeau successfully escapes, and Father Brown returns the gems to their owner.

“Your downward steps have begun. You used to boast of doing nothing mean, but you are doing something mean tonight. You are leaving suspicion on an honest boy with a good deal against him already; you are separating him from the woman he loves and who loves him. But you will do meaner things than that before you die.”

Three flashing diamonds fell from the tree to the turf. The small man stooped to pick them up, and when he looked up again the green cage of the tree was emptied of its silver bird.

The restoration of the gems (accidentally picked up by Father Brown, of all people) ended the evening in uproarious triumph; and Sir Leopold, in his height of good humour, even told the priest that though he himself had broader views, he could respect those whose creed required them to be cloistered and ignorant of this world.