Dulles to National at 4:30 PM or so is not a short trip. 66 is worse eastbound inside the beltway than westbound. Getting up the Parkway would require sitting in traffic in Tysons as all of the Marylanders start to head home. Best bet - take the Flyer to the Wiehle Metro stop and use Metro to National. Assuming you don't have too much luggage, that shouldn't be a bad trip and should take about an hour or so, and cost about $5.00.

4 hours or so is plenty of time to get between the airports. Its a question of how much you want to spend and how you like sitting either in traffic or on the subway.