Naruto Thread (Spoilers For Sure)

People assumed that it was the 4th Hokage, but as we later found out since his soul was sealed it would be impossible to do so.

Which would explain why the summoning failed. Yes, it was played off as the third putting his hands together somehow magically did something about it, but now that we know how the 4th died, it's much more likely that it failed only because of the death god sealing.