I don't know
I can tell you how it should look in theory, but if you ask me to put names there and make it a realistic back 4 for next year, then I can't make it.
Quality always comes first, so of course I wouldn't choose an experienced defender ahead of a non-experienced one just for the sake of it.
So if the defenders we're talking about are all quality ones, then i'd like a more mature player next to Chiellini. Someone who knows when to do things. When to make a foul, when not to make a foul, when to be dirty, when to do some defensive tricks etc.
For the full back positions i'd like players with great stamina, who are very good defensively and one of them to be good offensively too.
Now you put the names