- that last coffin was summoned, so its hard to say that nobody was inside :S Thats the reason why lots of us are confused about it; is it 4th hokage or not??
I still think that it was 4th Hokage, why? Orochimaru wanted to see how his former teacher and Hokage, Sarutobi ( known as the master of all jutsu ), was going to get KICKED by the previous Hokage's.. Orochimaru wanted to disgrace him in his own town with letting his ass whooped by 1,2 and 4 Hokage,.. all because Sarutobi didn't made him the 4th Hokage.
there would be no other body that Oro would summon at that moment against Sarutobi. Orochimaru had probably planned to summon 1,2 and 4th because they were all considered as the strongest shinobi's from their time AND THE FACT that he wanted to play a devilish mind play with his former teacher... summoning the previous hokages for disgracing him in his own village.
For whom would Sarutobi be so afraid for not getting summoned?? Yondaime was next generation and was probably stronger then Sarutobi in general. So when he saw 1 and 2, he thought okey.. but when he saw the last coffin ( assuming that it was 4 ) sarutobi was like; DAMN!! not him.. IF the 4TH hokage was summoned, sarutobi was gone in 10 sec with 1 and 2 next to Yondaime.
conclusion; I personally think that it was 4th Hokage, but how he could do it is still a mysterie. Maybe we think that we know how Edo Tensei works, but we probably don't..