Must one seek forgiveness and repent after jokingly calling someone a Kafir four years ago out of ignorance of the rulings?

It is not permissible to accuse a Muslim of disbelief (kufr), whether in seriousness or jest, for in the agreed-upon Hadith: "Any man who says to his brother: 'O disbeliever!' then it has surely returned upon one of them." And his saying: "unless it returns upon him" is understood to refer to minor disbelief (al-kufr al-asghar), which is a form of grave warning and an analogy to disbelievers, not actual disbelief. Scholars have considered labeling a Muslim as a disbeliever to be among the forbidden taunting (nabz) that Allah has prohibited in His saying: "Nor call each other by offensive nicknames." (49:11) And whoever does that, repentance and seeking forgiveness become obligatory upon him, and seeking pardon from the one he accused of disbelief, if that would not provoke his anger, because that is a means for expiating sins.