What does Genesis 40:4 mean?

Though he is still confined to the king's prison after several years, Joseph's reputation as a faithful and trustworthy servant continues to grow (Genesis 39:21–23). Joseph is here because of a false charge, brought by the wife of his former master, Potiphar (Genesis 39:11–20). When he was first introduced, Potiphar was named as the captain of the guard (Genesis 37:36; 39:1). Since Joseph was cast into prison, however, Scripture has only referred to the title, rather than a specific name. It's not clear if Potiphar is still in his position.

Whomever the captain is, he recognizes Joseph's value (Genesis 39:21–23) and appoints him to supervise two new prisoners. These are the former cupbearer and baker of Egypt's ruler, Pharaoh (Genesis 40:1–3). It's not known if the captain of the guard who appoints Joseph to care for these high-ranking servants is still Potiphar or another. We're not told how long Joseph has been here, but it has likely been several years, at least. Just as he's been there for "some time" prior to the cupbearer and baker arriving, it will be "some time" before anything else changes.