Their are alot of Youma's in japanese mythology, so talking about which one is stronger is pretty much just guesses, I doubt their will be more than 1 stronger than the kyuubi in naruto though.
Also did anyone else notice the similarities between Kyuubi and Inuyasha's father? :]

On the 3rd vs 4th debate, their both dead, and neither of them had strength as a prime objective, so the fight is not something worth talking about. And the fact that the 3rd coffin wasn't in the manga, leads me to believe, that the creator realized the 4th had his soul binded into death and couldn't be rebinded to his body?Not to mention the fourth probably would have just broke the jutsu and snapped ochimaru's neck ;]

Itachi would have been such a good 5th Hokage though ;] to bad hes evil.