4 Full days in Croatia. Which city/town will be ideal to go? (I know this is very little time)

I was in Dubrovnik last yr, it is wonderful!! A walled city with lots of interesting places and the Aegean sea to dine by and views. Take a water cruise for half or full day if u are so inclined. U will not be dissappointed.
Do not miss this beautiful city, it is a delight!

Croatia may not be as practical a stop in between Switzerland and Greece as its proximity to the latter might suggest. You might want to check flights, routing, and pricing. That said, Dubrovnik is an excellent choice. I think you could even do Split and Dubrovnik both potentially. I had to pick one, Dubrovnik. My wife and I were in both on a cruise in 2011, I could spend three or four days in either, not because the sights take that long to see but because the ambiance is pleasant and it would just be relaxing. The weather in October was great, not too hot but sunny and warm enough for us. I am sure you will have a great time.

Check the logistics on this as these places are not well connected. You'll spend 2/4 days traveling many hours.

I agree with the others about the logistics. Getting to Santorini will probably require that you go through Athens--which I'm guessing you plan to visit at the end of your trip, and that would be an opportunity to connect with another part of Greece. Perhaps Nafplion or classical sites? Or you could use the time to visit other islands in the Cyclades if they aren't already on your itinerary.

Croatia's great, but a quick trip between Switzerland and Greece isn't the way I'd want to do it.

I think that the only flights from Dubrovnik to Greece are into Athens, which is a lot of hassle for only 4 days. I'm not sure there are flights every day - check Skyscanner.

One approach would be to Google Santorini airport and see which destinations have a direct service and make your choice that way. Going via Athens from wherever is going to significantly eat into your 4 days.

Croatia is a fantastic country, but the logistics to Santorini may not work for you on this trip. Personally, I would pick Croatia a hundred times over Santorini, but each to their own!

If it were my itinerary, I'd go to Dubrovnik for 3 nights, spend a day exploring the old town, and a day exploring something outside of it. We had 3 nights in Dubrovnik at the beginning of this month and did just that. Our second day was spent in Montenegro. Beautiful indeed, but we'd do the border crossing differently. Take the lesser-known border crossing and avoid spending 2-3 hours crossing the border.