Let's break this down carefully, step by step.

Start with Jacob.

Jacob’s son → call him A.

A’s son → call him B.

B’s son → call him C.

C’s son → call him D (this is “the son of Jacob’s son’s son’s son”).

Now the question asks for the paternal great-great-grandfather of D:

D’s father → C

D’s grandfather → B

D’s great-grandfather → A

D’s great-great-grandfather → Jacob

Answer: Jacob