“For those who are familiar with IAD, is 4 hours long enough to get our luggage from the WN flight, re-check it, go through security, and get to our flight?”

The short answer to your question is maybe.

In a perfect world where everything runs on time and your suitcases from Denver actually make it to IAD, then yes, four hours is adequate to do what you propose. However, as others have mentioned, weather or maintenance delays, cancellations, or lost/late arriving bags can cause this plan to implode quite spectacularly.

If you do not have the option or desire to fly on a code shared partner from DEN-IAD (thereby offering some protection against misconnecting to your European destination), I would highly recommend flying to IAD the day before your flight to Europe. This ensures adequate recovery time should a flight be delayed, cancelled, or unexpectedly rerouted for weather (which is quite possible during the height of thunderstorm season in July). I fly weekly for work and have already seen all three scenarios play out this year.

Fly in the day before, get a hotel with free airport shuttle service, and sleep well knowing your European vacation will kick off without the worries of making it to IAD on time. Have a great time in Europe!