4:30 is the beginning towards middle of rush hour, in other words, expect to sit still for a while. The good news is you can use the toll road portion (exclusive use for airport business) for part of your ride which will help, the bad news is even though you are travelling in the "opposite" direction to rush hour, the DC area has a lot of traffic going all over the place and that isn't as much a benefit as it used to be.

Likely the taxi will take I-66 into the city, in which case you will hit traffic once you exit the toll road special lanes until the merge with I-66 is complete. That will be a very slow process, but it does move. I hope an hour and a half is the high side, I usually add 20-25 minutes to my ride when travelling through that area during rush hour. Note that if its bad weather, that the additional time goes up exponentially - "eek, there is a drop of rain, let me hit my brakes" is the rush hour mentality around here, unfortunately; and if it's snow, really watch out.

Because you are arriving at rush hour and you have luggage, you might want to check with a few local sedan services and see if you can get a good deal with them. They are usually flat rate, or charge by the mile and if the figure is close (your taxi cost will also be higher because of the traffic tie-ups), it might be a better solution.