It is a bit crowded to take in 7 countries in one month, but it's definitely doable. Travel distances, with the exception of within Poland, are not that great, so you won't lose too much time that way. We made a slightly different circuit in that region two summers ago - 6 countries in 5 weeks - and it was a good trip. Could have used more time, but that's always the case!
4 days in each city will give you a good sense of them, some you can spend less time in, of course. We had 10 days in Poland, split between Gdansk, Szczecin, Wroclaw, Boleslawiec and Krakow (been to most of them before, and don't care much for Warsaw, personally), 5 in Budapest, about 6 in Prague...fewer in Vienna, Bratislava etc. You can balance things out as your interests dictate.
I haven't been to Croatia or Slovenia, but both sound wonderful - just didn't work out last time round.
A lot of people on these boards know and love eastern Europe, so when you've figured out where you think you might want to go, or if you need some ideas, feel free to ask away - you'll get plenty of suggestions! It does help if you give an idea of what you like to do, see etc, because the region offers so many different experiences, and we all have our own ideas about what's "best"!