This is The Unsuccessful yet Academically Unparalleled Sage - A Cheating S-Rank Sorcerer's Post-Rebirth Adventurer Log.
From Baka-Updates:
After reincarnating in a different world, the great sage Ephthal spent all of his life researching magic. However, he who had aimed to reach the summit of magic, knew of the limits of his talents and fell into despair, thus closing the curtain to his lifetime. However, 400 years later, he reincarnated for the second time and enrolled in a magic academy using the sorcery and knowledge he acquired from his previous life. As if to sneer at the state of magic that had been in decline for the past 400 years, he easily invokes legendary class magic! With overwhelming power, the entire world will kneel before him.
At the beginning of the story, we're introduced to a wind mage named Ephthal, one of the four Great Sages of the Kingdom of Azul. After defeating the Demon King, alongside the other Great Sages, he's given the title 'Emperor of Thunder.'
As an old man though, he learns that he has no magical compatibility, which explains why he could never keep up with the other Great Sages, despite training more than any of them. He progressed as far as he did on the basis of of sheer hard work and willpower, rather than innate talent.
He later dies of old age and is reincarnated 400 years later as one of his own descendants. At the age of four, he regains memories of his previous life, and vows to become the strongest mage of his generation this time around.
He also discovers that even as a boy, he's just as powerful as he was in the prime of his previous life. In the second chapter, he's shown practicing magic with his older brother, and proves capable of casting much stronger spells.
