Just to reply to the very first posts in this thread about how we're supposed to know the 3rd coffin had the 4th in it:

If you looked at the coffins as they rose up, they were labeled 1st( ? ) (Im not familiar with the character but I'm guessing its another way to write 1st), the 2nd coffin had the number 2 written on it, and the 3rd coffin had the number 4 written on it before the Hokage got rid of it. Im pretty sure thats the key hint that the 4th was in the 3rd coffin.