Very much this. You've covered it really well. It's not like Vinny or Nico don't fit, it's just the other two fit more.Vinnie's a great defender, as is Nico. But I don't feel either of them fit as well into Pep's team as Stones and Laporte. Those two might lack a bit of the physical presence at the back, it's true, but offer a much broader range of passing, a lower error rate when taking the ball out of defence, and greater calmness on the ball: recovering the ball deep in their half after an option attack, who is more likely to put their foot through it and gift the ball to the opposition, and who is most likely to find a team mate?
But we are totally spoilt having all four.
The only thing I'd disagree with is that Stones and Laporte are lacking physically. Stones has shown on many occasions his strength when he's confident and Laporte is more than accustomed to that part of his game from his time in Bilbao.