Talk to the boarding crew before you leave. If you give them enough notice (and they're not in a rush), the captain may introduce himself, share a glimpse of the cockpit, and provide your nephew with his own set of wings (a few of the major airlines still give out pins to kids).
At 4, he might not appreciate souvenirs yet, but I've given slightly older nephews a thin 3-ring binder with blank pages to draw pictures of the plane and a clear page protector which is great for collecting boarding passes, napkins with the airline logo, (even a barf bag -- they get a kick out of that...). Have fun!