There not going in Chronological order. There's actually debate to what order you should read the books. The story of Magicians Nephew happened before the rest of the books, but in some ways you would appreciate it more if you read the lion, the witch, and the wardrobe first. I read the books in chronological order, I don't really care what order you read them, but some people debate about it.
Your right, Prince Caspian is I think the 4th book, but it's the next one with the Pevensie children, so they're probably making that one next so they can do it with the same kid actors before they get too old (and it kind of makes sense to continue the story of the Pevensie children, and then going back to the horse and his boy, rather than confusing the audience). Prince Caspian was also the 2nd book written, just FYI.
Now I hope that all made sense, :lol: I kind of typed this in a hurry because I'm on my way to class.