The Guide to the Magic for Kids has a lot of interactive stuff. My 6yo was able to write her thoughts about each ride/attraction and fill out the journal pages in the back (there are prompts to respond to).
A 4yo could definitely benefit from the book though...you would just have to ask him his opinions and record them for him.
There is a sticker for each ride/attraction on property (don't know if they have added Toy Story Mania yet though), close up pictures to search for in each section of every park, flags to color in for World Showcase and a monorail check list of all the colors that exisit.
We all definitely had a lot of fun w/ the book!