I guess this counts as a picture book, since it's a pop-up (why don't they put some pop-ups in a novel? Wouldn't that be AWESOME?! Why should the kids have all the fun?), but it's a very in-depth picture book. Probably not suitable for your 4-year-old.
My kids spent almost an hour going through it the first night we had it home, reading all the information behind the flaps, etc. It helps that the story of the Titanic is fascinating, but the book also simply does a good job getting and keeping kids' attention.