I have no experience but I would not count on there being a close car at that hour. The only way to be certain is to check it out on another weekday at 4 am. Uber is not a bargain either if you are relatively close like Potomac, so I would do Washington Flyer. If you get a black car then it will be a lot more than a cab. With UberX the guy might pull up in a Honda Civic.

I would seriously not get their earlier than a few minutes before 5 am if you are checking bags and have Pre-Check.