I had the concierge at my PRG hotel get me a ballet ticket when I was there last December--it was Nutcracker season and tix were tight, but she caught a cancellation. I had the impression it's usually easier. There are tons of chamber music recitals that are hawked in front of the astrological clock by people carrying placards. I think 4 days there is just about right. FWIW, I was told that September in PRG still sees a lot of tourists. Haven't been to Budapest, and last time in Vienna was 1967, so I can't help you there. Have a great trip!