Commentators question the sequence of Noach’s sons’ birth order. Who were the eldest, the middle, and youngest children? Several classics commentators have weighed in on this subject, bringing proofs from Scripture. Another particularly curious and quirky question is this one:

How many sons did Noach have? The quick answer is three but might there have been a fourth one as well? Fairly, students wonder: From where is this query coming? After all, neither the Torah nor Tanach make mention of a fourth son. True enough. However, some ancient historical annals not only do list and attribute a fourth son to Noach but they add substance to their claim by asserting that his name was “Unico” (אוניקא).
Q: Should the Tanach be considered as definitive in matters such as this one?
A: The long and short of it is a resounding: Yes.
But here’s a creative caveat.
If we examine the word “Unico”, we see in it a similarity to the Hebrew word: yonek.(ינק) meaning to nurse or nurture (as in a child that suckles from its mother). Furthermore, if we look into the Jewish tradition, we see that Noach was extremely close and fond of Shem’s grandson Eiver. In fact, the venerable and ancient mariner considered Eiver his very closest disciple.
Let us conclude. To be sure, Noach had only three sons. But on account of his bond and affinity with his great, great grandson Eiver, perhaps it can be said that it was like a father/son or Rabbi/disciple relationship. The Torah alludes to and the Sages teach that a man’s grandsons (or disciples) are commonly and affectionately called sons.


Excerpt from Abravanel’s World of Torah: Bereshit, Parashat Noach.
B.T. Sanhedrin 19.