Well, Superman was a quarterly? at the time.
So #4 was the Spring, 1940 issue (covering April, May and June, and expected to be on the stands for three months?).
Action was a monthly, so #23 was the April, 1940 issue.
I have the #4, don't remember the Luthor story. I don't have the #23.
But if Superman doesn't know who Luthor is in #23 (Luthor's introduction), and he knows who he is in Superman #4 (oh, this person_who_is_obnoxiously_self-impressed is back), it's clear that Action #23 was his first appearance and it was just by happenstance that the Spring issue of Superman hit the stand days before the April issue of Action.
Just speculating here.