My boyfriend and I, both in our early 30s, are looking to make a stop in Croatia, in mid August, but only have 3-4 days. I know the limited time won't permit too much, so while doing some historical sightseeing would be nice, we are more interested in exploring beautiful beaches and experiencing good food. Somewhere with a good, but safe, nightlife is also a must. Last piece of info: we are flying out of New York to Croatia, then would be flying from Croatia into Porto, Portugal after the 3-4 days. Thanks so much for your help!

With this amount of time, it is imperative that you do the legwork to see where you can fly to. Once you have done so, I would be glad to chime in on the relative merits of 3 days in Zadar or Split or Dubrovnik, but it seems silly for people to tell you where to go when you may not be able to get there before you have to turn around and depart!

Also make sure to also find out from where you can snag a flight to Portugal.

Thanks! I'll see if I can fly to/from those 3 cities.

Much as I loved Croatia, if your destination is Portugal, I trust you've considered stopping somewhere closer than Croatia. ;-)

That is not an exhaustive list of where you can fly to--that was my point, that you have to figure out the flight logistics. I would also stick to Portugal, but you will just have to see if it can work or not.

Biggish airports in Zagreb and Split, smaller ones in Dubrovnik, Rijeka, Pula, Losinj, Zadar.

In August Dubrovnik will be slammed with cruise ships and tourists. I'd skip it this trip. I'd stay in Trogir (which is near Split's airport) and make excursions into Split and along the coast.

But it all depends on the flights.

If you can spend more than 2000 USD/pax for this multi destination flight, you will have no problem.
If you want to pay only half of that, you will have to compare fares from NYC to several European low cost company hubs and combine cheap overseas flights with cheap European "domestic" flights.
A combination of Porto on the one hand with beach destinations like Dubrovnik, Split, Zadar or Pula on the other hand will never be cheap in high season, however.