But I'd rather have expectations of flexibility (e.g., take day off or a short day because didn't sleep well, or family activity, or enjoy the nice weather, or just finished a work crunch), than (I imagine) expectations that, on those 4 days, one had better be there the full day and at least pretending to work.

For some jobs, flexibility could mean that someone can arrange with their manager&team to personally have a predictable 4-day schedule.