Exactly.
But if we get Sampaoli as our new coach who has shown his preference to play 3 at the back in most games and he'll continue to do that here then we should bring another CB or preferably two in summer along with promoting and keeping Marlon in first team (so, not loaning him). If we go that way then I'd prefer one CB to be more experienced on top level. Not saying he should be old but somewhere in the 24-26 yo range not those 20 yo kids we are currently looking at. We can't go into the season with only Marlon and another unproven young CB backing up Pique, Umtiti and Masche (who could and most likely will also leave in next year or two).
Of course, another question is whether changing formation that drastically based on how our team is constructed and considering that most of our players are most comfortable in 4-3-3 would really be the best for us but that's up to the coach to decide. I think Robert should bring whoever our coach will be the players he wants as long as we can financially afford them. Yes, listening to coaches wishes didn't always turned out well but I prefer that way over spending money on someone that SD likes but coach will then never count on.