There Is a Bird on Your Head by Mo Willems
I happened to see this book when I was looking for a different one and picked it up because the title amused me and we've enjoyed books by this author and illustrator in the past. First Son laughed out loud the first time we read it, and continues to do so. It's a silly story and the humor appeals to four year olds and adults alike. I love the expressive faces the most! First Son has already memorized it and loves to read it to us as much as he loves for us to read it to him.
We picked up a few of the other Elephant & Piggie books at the library, and feel this is the best of the four. (There are two others you can pre-order at Amazon.) Highly recommended for the preschool crowd!