Itinerary advice for 4 1/2 days in Vienna - please comment on my tentative plan

Heuringen (wine taverns) for one or two dinners. Many are on outskirts of Vienna, but we found one centrally located in the RS Guide Book.

Coffee and pastry.

Belvedere Palace Museum, perhaps, if the weather does not cooperate with all your other plans. LATE NOTE … or if seeing Klimt’s The Kiss (and other art, esp 20th C) is important to you.

BTW, at the Kunthistorisches Museum, way high up among the tops of the columns of the Grand Staircase inside the museum, there are frescoes by Klimt (and his brother) that are a treat. We were fortunate to be there in 2018 when scaffolding had been erected for a show called Stairway to Klimt.

While at Schonbrun, visit the carriage museum.

As I recall, the Belvedere Palace is easier to get to than Schonbrunn; and I liked the art of the former better than that of the latter. The Belvedere also has beautiful gardens,
Half a day for the Kunsthistorische isn’t a whole lot. It might be a good idea to scope out the collection beforehand so that you can use your time efficiently. I’m guessing that there are on-line resources to aid you so that you will not miss the must-sees.

Thank you for these tips so far. As for the Kuntshistorische, I think it will be closer to 3/4 of the day. We plan to get to St Stephen’s at 9am opening and the audio tour takes about an hour. I think we’ll get to the museum before 11.
Ideally, we will have the opportunity to tour St Stephen’s on our arrival day, but the main part of the church closes at 4:30 (if I remember correctly). So if that doesn’t work out, it will have to be the morning of day 2.

There is a lot to see in Vienna and I think you have a pretty good plan. I've been there twice and have yet to make it to Schonbrunn or the Wachau Valley, believe it or not. Agree that the Belvedere is well worth it. Timed tickets for the Upper Belvedere are a good idea. The lower Belvedere can be skipped unless there is a good special exhibit there, check their website; when we last visited there was a great Klimt show at the Lower. One thought about limited time, consider doing either Schonbrunn or the Hofburg apartments, rather than both, as they are both Hapsburg royal palace rooms and thus perhaps a bit repetitious (with caveat that as noted above, I've only seen the Hofburg). The Treasury is well worth it, either way. St. Stephens is certainly worth a quick visit, but frankly I found it a bit underwhelming, I wouldn't put it in the top 50 of European churches or cathedrals I've seen. The best thing about it is looking at the multicolored tiled roof which you can do from the street outside. For something quite different from the usual palaces/art/music circuit in Vienna, they also have one of the best military museums anywhere, if you have any interest in that. The Hofburg apartments feature "Sissi", but how about another Hapsburg, also assassinated, who was much more historically important -- Franz Ferdinand's car and uniform with bullet hole in it at the military museum.

Maybe add the Music Museum that Rick mentions in his book. I think it's fairly close to the Opera House. The music at the Augustianian Church is sublime.