I've a four year old boy (well he will be in January) and I've just taken him on his first flight (to BCN - not too long / not too short). He enjoyed take-off (looked out the window the whole time), got bored in the cruise, but again enjoyed the landing (once more looking out of the window).
What I'm trying to say is that I speak with some experience of four year old boys (if you'll excuse the double-entendre')
For a stocking-filler, how about one of those little airline models of say a Go B737-300? Nice little thing to have on his bookshelf, or for him to run around the room with making all kinds of vrrrrooooommmmm noises!
But IMHO perhaps the best thing for a lad of that age are 'picture & word' books with lots of nice drawings of planes and/or airports (e.g you point to the cockpit and ask him who sits in there and what is it that they do they do, point to an engine and ask what it is, ask how many engines does it have, etc...).
For bed time a story book with lots of aircraft/airline/airport pictures is a MUST - guess what he'll be dreaming of ?!
Likewise, a simple (ie. 30 piece, not 1000) jigsaw puzzle with anything aeronautical on it. Young kids love these kind of things (once you can get them started on it there's not stopping them - and all the while they're learning reading, counting, motor-skills, shape/pattern recognition, etc), and
again all the while you can be asking them what the bits of the aircraft are.
Trust me mate, the old favourites really are the best - please ignore all the computer games, etc..
But best of all are books !
Finally, knowledge IS king !
Ps. Hence the edit - don't worry if they're not enamoured on X-mas day with what you've bought them, and instead the Mega Blaster, including serious bells and whistles and a real LOUD sound, that uncle Bob who works at the toy store gave them seems to be their favourite - it’ll soon be discarded - and some weeks/months later, after a little prompting, they will be delighted to show you just how much they now about all things aeronautical ! e.g. my lad now knows more about the B733 FMC than I do !
[Note: This message has been edited by CrashDive]