tHE 3RD HOKAGE took on the 1st and 2nd hokage in edo form and defeated them which i would say is harder since they could regenerate whenever they where damaged by hiruzen being the 1st and 2nd hokages former student he has displayed great knowledge of his former masters abilities and was able to counter them when he fought them. i dont think the 3rd will even have to resort to the death god real seal reaper< whatever its called lol> but its always there none the less
Now being Tsunade Former Sensai the 3rd would have great knowledge of Tsunades strengths weaknesses and abilities and didnt tsuande or jiraiya say something along the lines of we where never able to defeat him even when we all tried (All the legendary sannin jiraiya, tsunade , Lord Orochimaru) soo theres that.
now its just Hiruzen Sarutobi The god of shinobi the professor able to use all of the hidden leafs jutsu's VS the Yellow Flash Minato Namikaze
Having been former colleges with the 4th and advising him when they both where alive as hokage i believe That each would have a great Knowledge of each others abilities strengths and weaknesses.
i Believe that the 3rd will defeat the 4th due to age and experience.
Danzo was never Hokage he was the Hokage Elect Pending the votes from the villages Jonin since he was killed by sasuke he was never able to actually be given the title of Hokage