I will be traveling from the United States to Dusseldorf and have a four-hour layover before going on to Munich. During the four-hour layover I would like to be able to leave the terminal area to visit with a relative that lives in Dusseldorf. I am wondering how complicated or difficult it would be. Do you go through customs in and out? Also is there an area where one would be able to sit with a visitor at the airport and remain close by.
You go through immigration in Düsseldorf, customs is in Munich, provided your flights are on a single ticket. So, given that you need approx. one hour from landing past immigration and you have to go through security again one hour before taking off to Munich, you will have two hours to meet your relative. There are a couple of restaurants at the airport.
If you want to visit in a more quiet atmosphere you could meet at the Maritim Hotel which is attached to the terminal. Walk down the hallway towards the end, get on the elevator and you will be in the hotel lobby.
We have stayed at this hotel and they have a spacious lobby, multiple restaurants and bars. I don't know what hours your 2 hour visit would take place but this might make a difference in hotel restaurant/bar opening hours vs airport restaurant/bar opening hours.