4th Hokage would be the Strongest Ninja
The 4th Hokage would be the Strongest Ninja ever if he didn't die so young and before he hit his prime. I know a lot of Naruto fans might disagree but hear me out here now. The 4th Hokage (Minato Namekaze) managed to accomplish so much before 26. He died around when he was 25. He became the youngest Hokage and was feared throughout the ninja world. He was given the name yellow flash due to his immense speed and a warning was given to ninjas from other nations to run if they ever encounter the yellow flash in battle.
The Fourth Hokage single-handedly slaughtered 1000 ninjas from the stone village and ended the third ninja war. This feat resulted in him being picked to be the 4th Hokage over Orochimaru by the Third Hokage. He achieved all this while still being a very young man with untapped potential.
Minato was a very keen-minded individual, having the highest scores in the history of the Academy. In combat, he could correctly deduce the basic mechanics behind a technique after seeing it only once, and from there plan around its strengths and weaknesses to his or his allies' advantage. With his observational skills, he could determine an opponent's plans and motivations based on only limited evidence.
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The Fourth Hokage single-handedly slaughtered 1000 ninjas from the stone village and ended the third ninja war. This feat resulted in him being picked to be the 4th Hokage over Orochimaru by the Third Hokage. He achieved all this while still being a very young man with untapped potential.
Fourth Hokage Intelligence
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While facing Obito and the nine tails, Minato manages to display some top-notch analysis skills to deduce the secret behind Obito's technique and defeat him. While also figuring out that there was a bigger plan in motion and taking the necessary steps to plan ahead. He sealed the nine-tailed fox in Naruto with hopes of him taming and mastering its powers. This way the leaf village manages to keep its demon beast and maintain its power and influence in the ninja world, while Naruto gets to master a powerful beast to face the bigger plot of Obito's plan. In essence, we get to see Minato planning the next 20 years in like 10 minutes while dominating Obito, the nine-tailed fox, and protecting the village.
What the fourth Hokage could Accomplish if he was Alive
Rasengan Mastery
Minato had in his arsenal some of the most lethal Jutsu in the ninja world. He died before he could complete his mastery of all his techniques, for instance, his Rasengan the highest level of shape transformation. This is one of the most lethal ninjutsu techniques in Naruto verse. It's also the signature technique of Jiraiya, Naruto, and now Boruto. Minato did not manage to fully master the Rasengan to the point of adding his own element to it like Naruto did to create the Rasen-shuriken.
It is stated that the 4th had 3 natural transformations: Fire, Wind, and Lightning.
If Minato was still alive I believe he would perfect the Rasengan to its highest point. It's also not a stress to suggest that he would be able to develop other ninjutsu as powerful as the Rasengan.
Senjutsu
Minato was able to use Senjutsu as we get to know in the fourth ninja war when he was reincarnated. However, his mastery over it was not optimal. If he was alive he would most definitely master Sage Mode. He was considered to be one of the best when it comes to chakra control. Minato being able to attain a perfect Sage Mode would make his ninjutsu even more lethal and powerful. He would be in a class of his own. He would possess speed, power, and intelligence.
Conclusion
There are so many more points I can make but writing is annoying. However, I think I said enough for you guys to get the point I am trying to make. The fourth Hokage if he was given the chance to live to a ripe old age would probably be the most accomplished ninja in the Naruto verse. He would have done so purely with his own strength and intelligence. He was not from any big clans like Senju or Uchiha, he did not possess any Kekkei Genkai or special bloodline. Just once in a lifetime prodigy.