The problem I see is travel takes time, and Munich and Salzburg are a long ways from Paris. You'll spend one of your days going to either, and then you have three left to sightsee, and that's providing you don't have to travel back to Paris to fly out.
CORRECT. 4 days will pass too quickly if transport time is excessive. It's neither essential nor desirable IMHO to travel as far as Berlin or Munich. But you can still accommodate your and your wife's interests, and you can do that in GERMANY.
Suggest you take advantage of the direct high-speed train service that Thalys trains offer. A trip from Paris to Cologne by direct train takes just 3.3 hours. The trip to Aachen is even closer - 2.6 hours. Both places are easy to explore on foot. And both have sights nearby that make for optional and easy outings.
.https://www.seat61.com/trains-and-routes/thalys.htm
https://www.seat61.com/international-trains/trains-from-Paris.htm#Paris-Cologne
COLOGNE:
The Cologne Cathedral is Germany's most visited tourist attraction, hands down. It's a Gothic masterpiece and a UNESCO World Heritage Site packed with significant works of art.
Museums of all sorts attract art and history enthusiasts. At least one of those would be good for YOU - the NS Documentation Center, housed in Cologne's former Gestapo Headquarters.
AACHEN: lovely old town center, plenty of history, Charlemagne's former digs.
(I would probably rent a car for a side trip combining the dragon's teeth and Monschau.)
REMAGEN: Nice old-world Rhine River town south of Cologne and Bonn, easily accessible by train. Visit the "Bridge at Remagen" museum there, housed in the supports of the ex-bridge there. Maybe you've seen the George Segal / Robert Vaughan / Ben Gazarra WW II movie by the same name?
https://bruecke-remagen.de/?lang=en
MARKSBURG CASTLE: In Braubach, south of Remagen. Easy to reach by train. Intact medieval castle:
https://www.marksburg.de/en/circuit/#/
(I would combine a day trip to Marksburg with the visit to Remagen.)