Hi demosthenes63!
I have been studying your itinerary and knowing that you intend to use hired car, I can conclude that it is doable without any doubt.
According to the order of the places that you intend to visit I suppose that you are arriving to Zagreb from someplace and intend to depart by plane from Dubrovnik to Vienna.
The only thing that I consider worthy reconsidering as far as the number of overnights is concerned are Plitvice lakes. After you have taken the whole day for Zagreb exploration and after the overnight there, it would be the best thing that you wake up early in the morning and depart for Plitvice. It is a distance of only 137 km and roughly 2 1/2 hours of driving, so you can save a day (overnight) here and add it to Split, so that you see more in that area.
Honestly speaking, Plitvice lakes are wonderful place and the nature lovers stay there for days, but as far as I can see, you have intended to remain there for a day, so this option will be very satisfactory for you. The best option is to come early, explore the lakes and have an overnight there. Just one suggestion, please, try to avoid Saturdays for the travel on this portion. It is so unbearably crowded then.
In case that you reconsider your plans, and assign three overnights for Split, you will be perfectly able to explore Split, nearby gem called Trogir and on route from Plitvice to Split you should not miss Šibenik with its wonderful historic, old part of the town adorned with Cathedral of St James.
As far as day trip to Hvar is concerned it is a bit impractical if you plan to use catamarans. The fact is that the first catamaran departs at 11:30 and the last catamaran departs from Hvar at 16:15. Basically, this allows you even less then full four hours in the town. In case that you take your car on the ferry and use it on Hvar island, then it is much different circumstance. Here I have to explain that the ferries does not dock at Hvar (town), but in Stari Grad which is not within the walking distance to Hvar (town). Here I have an idea. You can take the car early in the morning by ferry to Hvar, explore the entire island and then proceed to the easternmost part of the island (place called Sućuraj) from where you can use the ferry to the coastal place called Drvenik and from there just proceed to Dubrovnik.