Here are some reviews of the Cryptological Museum:

http://www.tripadvisor.com/Attraction_Review-g60811-d117071-Reviews-National_Cryptologic_Museum-Baltimore_Maryland.html#REVIEWS

it is a surprisingly entertaining museum especially if you are interested in wartime history, or engineering. It is up near Baltimore, so you could tie it in to a visit to something else there.

American Visionary Art Museum would be neat but again maybe too much for the 4 year old.

http://www.avam.org/exhibitions/current-exhibitions.shtml