Oh you're totally fine. 4 hours is a leisurely amount of time to get from DCA to IAD. International arrivals at IAD is super fast if you have global entry; if not, you might have to wait in a very long line but you will definitely be fine if you have a 5 hour layover. Even if you're the last to deplane and you wait in a long line, you'll probably clear passport control within two hours of when your plane touches down and have 3 hours to get from IAD to DCA.