At the beginning of chapter 4, David is at an important transitional period in his life. Or as he states: "I passed out of a placid period into one where things kept on happening." It is these things that transform him from naive to relatively hardened and astute.
The first example of his naivety is when he talks to Axel—his "best friend among the grown-ups." A mature David would have been guarded about what he tells him—and he certainly would have been if he had understood the potential consequences—but he tells Axel everything about his telepathic abilities and conversations with Rosalind. Luckily Axel's instinct is to protect him. The second example of his naivety is that he and his friends have been saying their telepathic thought aloud, and this despite the fact that they all had a feeling that they would be in trouble if they were caught. Finally, the third example is that David believed that all people from the fringes would have "two heads, or fur all over, or half a dozen arms and legs."
His growing maturity relates to how he reacts to the situations above. After his uncle drills it into him that he must not tell anyone else about his telepathy, and he must stop saying the telepathic thoughts aloud, David immediately gets the others to promise to do the same. As David says after that they became more of a group. They suddenly all understood, and particularly David, the responsibility they had to keep their powers to themselves. Finally, when he sees people from the fringes, he notices that not only do they look like normal people, but one of them looks very similar to his father.
From there David gives examples, such as the government passing the oversized horse as normal, of how corrupt the system is. He realizes that to survive he can't trust anyone; not even his own father. At the end of the chapter his new found maturity is expressed when he refrains from telling Sophie about some parts of Ethics his father had taught him.
I did not talk much about this part of Ethics to Sophie . . . it had to be admitted that she did not quite qualify as a true image, so it seemed more tactful to avoid that aspect.