Nice post. I agree with everything you said and although I do believe the fourth was the strongest I put him behind Madara and The First strictly because of the Kyuubi and since Madara is still alive its safe to assume that he has gotten stronger since then. I put Sarutobi behind Oro and the other because as Oro said he most definitely was much weaker because of his age but I have no idea exactly how the battle would have changed if he was younger.

At most what could have happened is that he would have had enough strength left to pull out all of Oro's soul which I don't think could have happened since that would've drastically changed the rest of the plot if Oro had died then. Honestly I just haven't seen enough of Sarutobi for me to rank him higher than I did.