What is the ruling on not having paid the expiation (kaffarah) for my brother's diabetes for four years, and is anything incumbent upon me to atone for delaying its payment?

If your brother is permanently unable to fast, he must feed a poor person for each day of Ramadan throughout his life, as Allah Almighty says: "And upon those who are able to fast [but with hardship] - a ransom [as substitute] is feeding a poor person." If he is able to fast, he must make up for the ten days.

The expiation for neglecting to make up for Ramadan is only obligatory if he was able to fast in the last days of Sha'ban of the year he missed these days. However, if he was unable to fast at that time, then no expiation is obligatory for him.

The fidyah (ransom) or kaffarah (expiation) does not become void with the passage of time; it must be paid now. As for you, if you delayed the expiation without a valid excuse, then you have transgressed against those who are entitled to it, and you must seek forgiveness. If it was due to an excuse, then there is no sin upon you.