I am going to pick two ideas that start to become clear during chapters three and four.
The first idea that begins to become clear is the idea that the Greasers and Socs are not all that different. It's during chapter three that Ponyboy and Cherry really begin to discuss the two gangs. Ponyboy feels that the two gangs are nothing alike. He expresses his feelings that the Socs seem to have everything easy, but Cherry explains that they have problems too. From that point on, Ponyboy is able to identify the key difference between how both groups feel.
"That's why we're separated," I said. "It's not money, it's feeling -- you don't feel anything and we feel too violently."
The other idea that becomes clear is that the conflict brewing between the Greasers and the Socs is going to come to violence sooner rather than later. Ponyboy and Johnny are walking the two Soc girls home, and they are seen doing it by the boyfriends. A fight almost happens, but Cherry and Marcia are able to prevent the fight from happening. It's clear to the reader at this point that the Socs are not going to forget the matter, though. A fight is coming, and it begins in chapter four with Pony and Johnny being attacked. Johnny defends himself and kills Bob. This only serves to further increase tensions between the Greasers and the Socs. A big fight is going to come.
It becomes clear in Chapters 3 and 4 of The Outsiders that something violent is going to happen to Ponyboy (and maybe others) at the hands of the Socs. There is plenty of foreshadowing previously and while the boys are at the drive-in. Pony is attacked early in the novel, and we find that Johnny has been severely beaten before. The Socs threaten them when they are walking the girls home, and the mobility of their Mustang gives them the advantage of searching and stalking the greasers. As the two smallest of the gang members, they are easy prey. Pony's and Johnny's fears grow, and when Pony has an argument with Darry and heads out in the middle of the night, the reader knows that something else is bound to happen.