Mr. La Marshmallow: I have limited time to use the computer in what passes here as a library. (L.Ron Hubbard, anyone??). But I will do my best for the good of the group:
Yes,, I've been to Calabria a few times but with four extra days I would definitely not recommend traveling all the way there--for four days! And you would be best renting a car for Calabria; it's possible by train but the beauty isin the rural areas, the Pollino and Aspromonte parks, etc. The cities, not so much.
I imagine Matera is in your Puglia plan (??). If so, soldier a night in Tursi. And the abandoned town of Craco.
There are SO MANY interesting and beautiful towns in Puglia that are not on the well-trodden tourist trail of Monopoli, Ostuni, Trani, Alberobello, Locorotondo, Martina, Lecce and Otranto. Are you planning to visit all of those?
From Trani, Castel del Monte and the area around there.
The Salento--towns like Nardo and Galatina.
Gallipoli and the Porto Selvaggio Reserve
The pottery town of Grottaglie
Taranto for its archeological museum (I've never been, though)
Gravina and Altamura
And the gorgeous Gargano promontory with the white town of Vieste (best with car)
If you let us know where you are already planning to go, we can help you add spots in Puglia.