Bone is fantastic - we've probably worn out 3 copies of it. Maybe not for the 4 year old set as the story gets pretty dark as it moves along.
One of my favorites from when my kids were little is Owly. No words - just amazing artwork and stories that are great for opening up dialog about what the characters are experiencing. Plus the author is a fantastic guy, my oldest and I would go see him at SDCC every year as a tradition, and ended up commissioning a print from him for her HS graduation.
There's some free PDF's up on his website, and the books were recently republished through Scholastic.