Jem says there are four kinds of folk, but Scout says there is one kind. Which view is correct?

I'm assuming the rest of that question is "correct" or "accurate". This is a highly subjective question that really connects to your own value system or prejudices. As for myself, I agree with Scout. The exchange between the siblings is as follows:

You know something, Scout? I've got it all figured out, now. I've thought about it a lot lately and I've got it figured out. There's four kinds of folks in the world. There's the ordinary kind like us and the neighbors, there's the kind like the Cunninghams out in the woods, the kind like the Ewells down at the dump, and the Negroes.

Jem is attempting to understand the prejudice he sees surrounding him. His division of people into four groups accounts for the hatred and discrimination he witnesses everyday. When Scout replies "Naw, Jem, I think there's just one kind of folks. Folks," she is revealing her own understanding of the world around her. she is not attempting to understand prejudice; she is accepting that it exists. this allows her to see that all people are one, no matter their background. But Jem doesn't agree:

"That's what I thought, too," he said at last, "when I was your age. If there's just one kind of folks, why can't they get along with each other? If they're all alike, why do they go out of their way to despise each other? Scout, I think I'm beginning to understand something. I think I'm beginning to understand why Boo Radley's stayed shut up in the house all this time . . . it's because he wants to stay inside."

Jem is worried by Scout response, because it basically means that there's no logical reason for discrimination based on race or class. This contrasts with his view of the world as a logical place. That is why he is so upset at the end of Tom's trial: Logically, Tom should be found not guilty. Jem is very sensitive to how people treat each other, & his connection to Boo Radley shows that he understands more every day.