妙法蓮華經玄贊 8.255

8.255

贊曰：四頌為四，一常為我子、二為諸佛子、三陳密行、四說勝因。此初二也。佛有三子，此大云長。

The Eulogy states: The four verses comprise four [topics]: first, always being my son; second, being the son of all buddhas; third, explaining his secret practice; fourth, stating his superior cause. This refers to the first two. The Buddha has three sons; this refers to the eldest, called the first-born.