In my current company, the start time is fixed (9) and adherence to it is flexible (probably also a 4 hour window, but with a strong unspoken preference for being reasonably "on time") so our work schedules overlap. I think this is a fairly efficient balance, and I would guess shouldn't suffer heavily from the problems mentioned by the GP if we were to introduce a shorter work day.

On your point on remote work, I don't really think this is true. One day response turnarounds can be a real pain in writing.