At that time of day, security at IAD will likely be clogged by pax departing on the first bank of international departures for the afternoon/evening.

The screening layout is also such that you benefit very little by using the elite line in such circumstances, as it only buys you a shorter line to the TDC--after that, the lanes themselves are a scrum with everyone else.

The Global Entry office is at the far right end of the main terminal (completely opposite the UA counters at the far left), right outside where international arrivals exit the secure area. You could either go upstairs to departures (and back down) to the security checkpoint at that end--which I rarely use, so I can't comment directly on its lines at that time of day--or stay on the arrivals level and go about halfway down the terminal to the Dulles Diamond lane. The Dulles Diamond lane may be shorter, but will almost certainly be filled with wheelchair pax and be using the NoS.

If you're able and arrive with plenty of time to spare before that 4:00pm appointment, you could try doing a walk-in. At worst, the appointment shouldn't take more than 20 minutes (watch a 6 minute video, get your fingerprints, and if doing the RFID card, your photo taken, too). Still cutting it awfully close...