Who's inside the third coffin that Orochimaru tried to summon?
As can be seen on this image, it was indeed the 4th Hokage that Orochimaru tried to summon.

  

The kanji that appears in that coffin is 四, which is the kanji for 4.  

However, unlike you said in your question, Hiruzen did not stop this coffin. It was the fact that the 4th Hokage was sealed with the Shiki Fuujin that prevented that summoning.