My own theory is that Nico (son of Aristotle) asked Aristotle to write him a book explaining Ethics.

Aristotle didn't start from stratch, but gathered a lot of his already written out works (whom he penned himself) and told his "scribes" to copy them in one go, neatly, word for word so Aristotle could give it to Nico. Later when Eudemus asked for a book explaining Ethics. Aristotle did the same (thinking Eudemus might enjoy a diffrent order) so he told his scribes to re-write Book IV as Book V for Eudemus.

So my thought is that Aristotle used his scribes as copy-paste. But wrote both books for his students.

My own theory is that Nico (son of Aristotle) asked Aristotle to write him a book explaining Ethics.

Aristotle didn't start from scratch, but gathered a lot of his already written out works (which he penned himself) and told his "scribes" to copy them in one go, neatly, word for word so Aristotle could give it to Nico. Later when Eudemus asked for a book explaining Ethics. Aristotle did the same (thinking Eudemus might enjoy a different order) so he told his scribes to rewrite Book IV as Book V for Eudemus.

So my hypothesis is that Aristotle used his scribes as copyists, but wrote both books for his students.