We have four nights between Zadar and Rovinj (where we want to stay and then drive through and tour the Istria area). Our plan was to spend 1 night in Zadar and then 3 nights in Rovinj to explore that area. Is that a good split or should we spend another night in Zadar? We will be there toward the middle of May. Thank you.
At you stopping at Plitvice Lakes National Park in between? If not, I'd take a night from Rovinj for that - or at least do a long day trip detour, even though the park gets busy by mid-day with tour groups, so staying over nearby lets you get into the park first thing before it gets crowded. May not be so bad mid-May, when I was last at Plitvice. Crowds weren't awful at mid-day but it certainly wasn't empty.
If you really don't want to stop at the park (one of Croatia's highlights), then I guess I'd ask what's on your agenda for Istria? I adored Rovinj, but it's tiny and can be explored quickly, in a few hours. It makes a great base for exploring Istria. You could do some hill towns on one day...and many people do a day trip to Pula on another day. I did both of those and thought the hill towns were interesting but could have done without Pula, which just didn't charm me. (I'd seen better Roman ruins in Italy.) Zadar was interesting and has its fans too but also didn't really grab me (I had a night there). So I guess I'd lean toward three nights in Rovinj...unless you had no interest in Pula. Then it's hard to say!
Porec, though as touristy as Rovinj, has a different look, and I enjoyed several hours there just wandering around. The tiny town of Vrsar is worth a quick visit, too.
I agree about Plitvice.