A It is. But while you may not have a prodigy on your hands, you still ought to be proud -- you have a kid who loves books and is ready to learn.
No doubt the book she's "reading" is one you've read to him over and over. She's associating the pictures on the page with the words she's used to hearing. You might have noticed this when she was a little younger and she reached to turn the page just as you finished the words. When kids are around 4, they have the memory and the language skills to say the words they know go with each page. Some kids can recite a whole book even though they can't really read.
Still, this is a big step toward reading: Your child will soon be able to recognize words and connect sounds with letters. All you need to do is keep reading together and mix new books with old favorites.