Who does Ponyboy blame for everything that goes wrong in chapter 4? What is Dally's reaction to this and how does it change Ponyboy's perspective?

Chapter 4 is the chapter that has Ponyboy and Johnny getting attacked by the Socs. The rival gang is drowning Ponyboy, and Johnny kills Bob in order to save Ponyboy. Johnny tries to take a lot of the blame in a later chapter, but Ponyboy doesn't accept that at all.

"This is my fault," Johnny said in a miserable voice.

While Johnny might have killed someone, Ponyboy doesn't blame Johnny for anything. Johnny acted in defense. Nothing would have happened had the Soc gang members not attacked Ponyboy and Johnny. The Socs came looking for a fight too because the park was in Greaser territory.
Dally's overall reaction is a reaction of calm and calculating. Dally is no stranger to breaking the law, and he's actually proud of Johnny for what he did.

He didn't bat an eye when Johnny told him what had happened, only grinned and said "Good for you" when Johnny told how he had knifed the Soc.

Because of Dally's experience with breaking the law, he immediately understands the severity of the situation that Ponyboy and Johnny are in. Dally knows that "murder" is a word that is now going to be linked with the boys; therefore, the boys have to hide out for awhile. Dally's use of that word and seriousness indicate to Johnny and Ponyboy that they could be in deeper trouble than they initially thought.

In chapter 4, Johnny ends up stabbing Bob the Soc to death after his gang attacks the Greasers and tries to drown Pony. The Socs attack Johnny and Pony as they rest at the park. The park is in the Greaser’s territory. Everything happens so fast that Pony realizes what has happened after he sees Bob’s body on the ground.
The boys are forced to run. Pony blames the entire incident on the Socs because they were trespassing and looking for trouble. Johnny had no choice but to react in self-defense and save Pony from being drowned. However, after they meet with Dally, he reminds them that the situation they are dealing with involved murder. Although Dally reacts calmly, both Pony and Johnny understand that they are up against a very serious issue. At that point, Pony is worried that he might not see his brothers or friends ever again because those involved in the incident will have to run and hide for the rest of their lives.