I think the 4th was not exceptionally powerfull (compared to other hokages). The reason it was him who defeated the monster fox was because he was the hokage at the time and it was his responsibility. Who ever went up against the kyubi was going to die for sure, and as he was the current hokage, he was the one that had to do it.

The 3rd said that he wanted Orochimaru to become the 4th, but he couldn't choose him. Orochimaru was the strongest but he was just so evil. He doesn't understand the point of being Hokage. Sarutobi chose the 4th because he loved the village (and was still a big time ass beater).

Now, to the topic...

Orochimaru says in the manga quite explicitly that Itach is stronger than he is.

If Itachi is stronger than Orochimaru, and Orochimaru is stronger than the 4th...

I think Itachi would beat the 4th's ass - then the 4th would seal him with his soul seal technique.
They would both die.