I'd also offer up a couple things - if you have limited time, take the Speedboat Tour which is a hoot (though could be too intense for a 4 year old)
Also, right next to the departure for that is the Sea Air and Space Museum where you can walk through a sub, as well as see the rest of the museum (which is on an aircraft carrier). There is also a space shuttle there, but not sure about that as it was not open to the public when I went.
We also did the Rockefeller Center Tour for a great look at the architecture, art, sculpture and history of the building... and a glimps into the cold war era and politics to boot. Even if you don't take the tour, go to the Top of the Rock Observation Deck as it has a better view than the empire state building (IMOO) - as there are two major NYC landmarks you can see from the Rock that you can't see from the Empire States building... namely the Empire states Building, and Central Park!

-Rob A>