I’m planning our next trip to in late September/early October, 2026. We are avoiding the Octoberfest in Munich so this is what I’ve planned so far. We have travel to Europe many times but not to these countries. Not including travel time I am thinking

4 nights in Budapest
4 nights in Vienna
3 nights in Salzburg
3 nights in Munich

We are active older travelers and enjoy having a slower pace where we can soak in the architecture, culture, arts and food scenes. Would you suggest changing the itinerary based on your experience?

That seems a very reasonable plan/allocation of time. If one were doing any day trips I might switch one night from Salzburg to Munich -- but that is just a minor quibble and the 3 / 3 you have is fine.

Your itinerary looks well-balanced and not rushed.

Does your itinerary leave you with?

3 full days in Budapest
3 full days in Vienna
2 full days in Salzburg
2 full days in Munich

If so, that seems reasonable, though I might drop Munich and spread the days over Vienna and Salzburg, or a stop in-between if you are traveling by train. (Just my opinion. Munich never intrigued me the way the other three destinations had). I'm assuming you are traveling in mid-to-late September? There will likely be many grape harvest festivals (at least in Budapest and Vienna) to enjoy.

I wanted more time in each city. For my interests: 4 days (5 nights) in Budapest (particularly if you are recovering from jet lag while there), 5 days (6 nights) in Vienna, 3 nights (2 days) in Salzburg, and 5 nights (4 days) in Munich. That said, it really depends on what you want to see and experience. You would, I think, do well to plot things on a calendar.