We visited Vienna, Salzburg and Ljubljana in December. we had 10 days and it wasn't nearly enough.
Start by figuring out how you will get from Croatia to any one of those locations. Also, as someone asked, where is your point of departure?
Have you traveled in Europe before? How do you plan to travel from one location to another? Where in Croatia do you disembark?
Let's assume it will take a day to get to the first location, which would logically be Ljubljana. I know it takes a full day to get to Vienna from there. I looked and it would be another day to travel from Vienna to Prague. Then if none of those are your departure location, you'd have to travel another day. I you went straight to Vienna, subtracting the day to get there and the day to travel to your departure location (eg Munich), that would give you 4 days on the ground. You could barely scratch the surface of Vienna in 4 days.
I suggest you rethink your plans. In attempt to see it all, you'd end up seeing nothing.