Paris to Germany:4 nights-Baden-Baden, short Rhine River cruise, Rothenburg and Munich Oktoberfest?

If you rent a car in France and drop it in another country there will be a huge cost.
You could take the train to Strasbourg, France, spend some time there, then take the train to Baden-Baden (do the Black Forest -Triberg), then rent a car and visit Switzerland, I recommend Interlaken and Lucerne.

Then head back to Germany, stop and see the Bodensee, then to the Romantic Road. I suggest picking it up at Augsburg, then visit the walled medieval towns of Rothenburg ob der Tauber, Dinkelsbühl and Nördlingen.

I do not recommend Stuttgart as a base. Stuttgart is known by the Germans as Stau city. A stau is backed up traffic.
Also, most of what Stuttgart has to offer is the Mercedes and Porsche factories.

Don't miss Heidelberg. The best Rhine cruises are from Rudesheim.

With only 4 nights I would take the train to Strasbourg, get a car and tour the Black Forest and Alsace

Thanks for the feedback.
For more details - after 4 day Germany visit we plan then to Lucerne or Interlaken for 3 days for Switzerland highlights. Then we re-enter France to Lyon for 2 weeks for South of France, Nice loop, thru Bordeaux/Loire then return to Paris for flight home.

I agree that taking the train to Strasbourg would be best. That is a charming city and you could spend the day (or a couple of days) there, then take the train over to Offenburg, and spend the rest of your time in the Black Forest. You do not need a car there, as the trains will get you everyplace; although if you did want a car, you could easily rent one in Strasbourg or Offenburg (a town in Germany near the border. You could even head directly to Gengenbach (see below), and spend 4 nights there, and include a day trip to Strasbourg while you are there. That's what I did, and it's a very easy jaunt over the border. If you do rent a car, just make sure you drop it off in the country where you picked it up from. So if you rent in Strasbourg, drop it off there, then take the train to wherever. Same thing with renting in Germany.

Gengenbach is one of my favorite places in Germany. I stayed here for 4 nights in 2022 and absolutely loved it. The town is very charming with beautiful half-timbered houses (it's frequently called a "chocolate box" village) and has a beautiful Altstadt, with quite a few restaurants and cafes to choose from. There are lots of things to do there and it's very quick (and easy) 40 minute train ride from Strasbourg.

Not far from Gengenbach is the Vogtsbauernhof, an open-air museum that is well worth a visit. It's a short trip by train from Gengenbach. And if you stay in Gengenbach (or any other town in that region), you would be entitled to a KONUS card, which gives you free public transportation throughout the area.

Then just get a train from there to Lucerne, and head on to the Switzerland portion of your trip.