If I buy a first class ticket from Prague to Vienna but there are no reservations available for a peak travel day, will I be standing for 4 hours?
If all the seat reservations have been sold, I guess so.
I never experienced that seat reservations were sold out. But anyway, you should buy your ticket and seat reservation not at the latest moment. There is no need to buy a first class ticket, in my view, but it will be certainly less crowded than 2nd class on a peak travel day.
How do you know when it's a peak travel day? We plan to make the same trip...