You won't need to worry about seeing the city early in the day before it gets too hot if you are going in April. In addition the water will still be quite cold. More like a mountain lake than the Mediterranean. Since it is so early in the season you will not have to worry about tourists and you'll really get a feel for the town.
There is a great bar outside the walls of the city on the cliff overlooking the sea. It is hard to find, look for signs painted on the walls that say "Most Buetiful View" or something along those lines with the mis-spelling. It is nothing but a few tables and a fridge to keep the beer cold but the view is spectacular and it is a nice quiet place. The owners are a couple who live near by and the hours depend on a combination of business, weather, and what they feel like doing.

I agree with Letiole's recs. in Addition, Albania is not worth the trip. Montenegro is also very beautiful, especially Sveti Stefan, Kotor and Budvar on the coast.

4-5 days is not long for such a beuatiful area so you will have to be choosy. I would say not to go to Sarajevo because you will lose a day travelling. Stay on the coast and enjoy the Dalmatian Riviera