4 days to fill in Germany

You could take a train south to the Eifel region to stay in the beautiful, castled town of Monschau. You could also visit the historic, larger city, Aachen on your way to or from Monceau.

You’ve probably been to cute cities on the Mosel but if not take a look at this easy train ride from Düsseldorf with interesting Bonn and Koblenz on the way.

You are also very close by train to some charming small towns in the Netherlands.

Go to the Münsterland.
https://www.muensterland.com/en/tourism/orte-muensterland/historic-town-centres/
https://www.muensterland.com/en/tourism/orte-muensterland/places-on-the-100-castles-route/

I loved Gengenbach in the Black Forest area. It was the most charming town and there was a lot to see and do in the area. It might be a bit more than three hours, depending on the trains – perhaps 3 1/2? It was very nice, though, and I really enjoyed my four nights there.

Oh, I’ll also echo Russ’s recommendation for Münster. I was just there yesterday and it’s a charming city. I’m actually staying in Ostbevern, which is about 30 minutes out, researching some family history, and I love the area. It’s great if you like bike riding or hiking. Very laid-back and nice. And there is a lot to do in the city, including some great cathedrals and churches and history.

Problem with going to someplace like Gengenbach, is that in order to get there in 3 3/4 hours from DUS, it requires using ICE or EC trains which are pricey based on Flexpreis tickets. I would not buy discounted Sparpreis or Supersparpreis tickets for this because you cannot be guaranteed your plane will arrive on time.

Better would be to choose a destination that can be reached with a NRW ticket or a VRS ticket. Then you can take any regional train to get to your destination. Also, be careful about the monorail needed to get from the airport terminal to the DUS train station. It requires a local short trip ticket to ride. That ticket will be included if you use an NRW or VRS ticket, and also Sparpreis or Flexpreis tickets, but not with a Supersparpreis ticket.

Other than that, I really like DUS airport. Not the zoo that is Frankfurt airport.

You have a lot choices within that 3 hour radius from Düsseldorf and "quaint, small areas"

I heartily suggest two that would that travel constraint: Go east to Minden an der Weser or southwards to Marburg an der Lahn. For both places you only need to transfer once. My first time in Minden was seeing it as a day trip from Düsseldorf.

A closer place is Soest/Westf., the oldest town in Westphalia....all these places are pleasant, tranquil and interesting, historically and culturally, am well familiar with these three.