I agree that the ceiling of 4h is correct for high intense cognitive activities and that morning, after few hours after woke up, is the best time for these activities. And I would add that trying to do more the same day will affect the next day.

Meetings, phone calls are distractions, especially in the morning, they could also bring different thoughts far from current task.

To have the best result we must reorganize the company according to this, because most organizations prioritize visibility over results, but compensation, promotion and trust structures reward deep work instead of meeting attendance.