It will be close as I-66 can be backed up starting at 2:30 before it becomes HOV only. That does not include any traffic on Constitution Ave before the bridge.
If you have your BP and get to IAD by 4:45 PM you SHOULD make it, use the diamond line on the lower level or the premium line and dont stop for food/newspapers.
It will be close.
Worst Comes to worst there is a 7 PM flight and a 9:45 PM flight to SFO.