To be fair to your employer, he may have conflated two different kinds of 4-day work weeks.

The current discussions are mostly about 32-hour weeks, but there is a very long history of what labour law calls the "modified work week" in which the number of hours per day or days without breaks are changed to allow for alternate scheduling without triggering overtime. I've worked 4-10s, 8 on 6 off, and other oddities since I entered the work force in the early 1970s.

The most common of those is 4-10s, and it's always been known by that name (4-10s) or 4-day week, or "4 and 3", with "4-day week" being the most common in my experience.

I know that my own following of this issue makes it clear that there are a lot of people confusing the two different kinds of 4-day weeks.