I think you missed the point of my post man, I was only mentioning Tsunade as a brief example, I didn't really mean anything seriously by it.
And there's absolutely no proof that the 4th was stronger than the 3rd. We can only guess.

A hokage won't pick a successor because he/she is stronger than him, but because they are the best candidate available at the time. After the 4th died, the 3rd hadn't picked a new candidate because there was nobody available who fit the position. I can't prove it, but I'm willing to bet that if the 3rd was a young man, he could beat the 4th.