In To Kill a Mockingbird in Chapter 4, why was Scout in danger and what kind of danger?

Jem thinks that Scout is in danger because she eats gum she found in the Radley tree and when she rolls in the tire. 

The first time Scout seems to be in danger is when Jem sees her chewing gum and asks her where she got it.  As gross as it may seem to use to chew someone else’s gum, Jem is more concerned with where the gum came from—a tree on the Radley property. 

“Spit it out right now!”

I spat it out. The tang was fading, anyway. “I’ve been chewin‘ it all afternoon and I ain’t dead yet, not even sick.”

Jem stamped his foot. “Don’t you know you’re not supposed to even touch the trees over there? You’ll get killed if you do!” (Ch. 4)

The neighborhood children are all afraid of Boo Radly and the Radley place.  They won't even eat pecans that fall off of the Radley trees, because they think they are poisoned.  Jem can't imagine why Scout would find gum in a tree on the Radley lot and chew it.  To Scout, gum is a real treat, even someone else's. 

Jem is not as concerned when later they find the pennies in the same tree.  These are two old Indian-head pennies.  Jem is excited about the pennies because he says that are strong good luck.  They wonder who might have left them in the tree, because adults do not have hiding places.  They decide finders' keepers.

Although the gum was probably not dangerous, Scout was in a bit more danger when she rolled in the tire.  She could have been seriously hurt when she got into the tire and Jem pushed her across the road.  

The tire bumped on gravel, skeetered across the road, crashed into a barrier and popped me like a cork onto pavement. Dizzy and nauseated, I lay on the cement and shook my head still, pounded my ears to silence, and heard Jem’s voice: “Scout, get away from there, come on!” (Ch. 4) 

Jem is more concerned that the tire stops next to the Radley place than for Scout’s safety in the tire itself.  They still think that the Radley place is dangerous, and are afraid to go anywhere near it.  Scout might have been hurt falling out of the tire, but Jem just tells her to get away from Radley gate.