okay, so maybe this isn't a very good book for four year olds to be reading (it does have decapitations after all). I seem to remember being about eight years old when I first read it, that seems mature enough. 13-15 year olds are probably (hopefully) beyond this reading level, so scratch that. I wouldn't buy it for my six year old nephew, but I wouldn't buy it for my thirteen year old cousin either.
Think about it though. Most kids who read stuff like this LIKE to be scared. I read it and it kept me up at night. And all night long I thought to myself "why on Earth did I read that." But then I'd get up the next day and read another story, or read a similar book. It's the thrill of being scared.
And this book was a nice introduction to horror. Oddly enough, the story of the beheaded girl didn't chill me, but the story TEETH did. It just really freaked me out, guys with ridiculously big teeth, now that's spooky. As for being concerned with a child talking to strangers, well this story illustrates the DANGERS of doing such. So I don't see any harm there.
Anyway, overall I'd say that this book is pretty solid for a second or third grader, but not a kindergartener. Basically it should be viewed on a case by case basis. If you think your kid can handle it, and they're a fan of this sort of thing, then by all means get it for them. And if they have a nightmare, be a parent and console them. That comes with the job.