Actually, that video covers Swiss cities, which are not the most beautiful places in the country.
He has an older video on the Berner Oberland; if I had only four days for Switzerland, I'd spend them all there, in Mürren or one of the other towns there. Here's that video: https://www.ricksteves.com/watch-read-listen/video/tv-show/best-of-the-alps
You would take a series of trains from Germany to Mürren, and when you leave, you take a series of trains from Mürren to Salzburg. These rides may be long. It will always be a series of trains because due to the mountains, the special kinds of rail lines, and how they run, you will have to change trains several times. Don't worry - all the connections are coordinated.
To find train routes, use Swiss Rail: https://www.sbb.ch/en/home.html
Don't try to "cover" the country - just stay put in one place. That way you have some protection against the weather. Mountains make their own weather, and mountain weather is both unpredictable and rapidly changeable. By staying in one place for 4 days, you maximize the chance that you will have clear weather for at least some of the time. The Berner Oberland area does have some cloudy day and rainy day things to do, but it's at its best when it's clear.