I personally believe it might still be three games. Sure, going by sheer length, Midgar is nowhere near as long as a third of the entire game, but it’s important to keep in mind that Midgar was the densest part of the game with plenty enough room to expand it the way the remake does. So going by the original story pacing alone, I think there’s just enough stuff there to put the rest of disc one in part 2, and then part 3 could be the rest of the game. Your four-part plan seems to be pretty possible too, though.

But of course, all this speculation is not taking into account the fact that the game might go completely off the rails at some point. I honestly expect this to be kind of a Rebuild of Evangelion situation - first part is pretty straightforward until the very end where there’s a pretty big change that means a lot for returning viewers but not a lot to new ones, second speeds things up and condenses they old story more while also totally taking it in a different direction (so in our case and the scenario I listed, we could for example reach the City if Ancients by the end of part 2 but Aerith doesn’t die this time and instead something insane happens), and then from the third part onwards the story is pretty much unrecognizable.