we travel with our 4 year old pretty frequently too, and were running into so many hassles we just gave up trying to figure everything out.
we ended up getting him his own mn state id card (yes, for a 4 year old) just for traveling. it was worth the $20 or whatever to not get hassled about "which one of you is chase" when there were 3 of us traveling, clearly the 2 adults had id's & the child did not. plus, chase loves having his own "license." :) we always assume he has to do everything we have to - take off shoes, take off jacket, etc. we always pack an empty sippy cup (just in case, since he normally does fine with the standard issue airplane beverage cup) and dry snacks (pretzels, goldfish, etc).
good luck - i know how frustrating it is to travel with a little one period, let alone get hassled at security!
J.
mom to chase (4) & paige (due 6.2.08)