Yes, I tried to fill in the actual work of the resource on a day by day base in the calendar form on the right side of the task usage or resource usage view. Setting 0 hours in one of the days and increasing the number of hours in the other days makes MSP calculate the actual duration correctly (4 days in my example). This works fine for fixed unit type and fixed work type (not fixed duration which is normal), effort-driven or not does not impact here. This is a work around and takes much more time to complete. Not sure if I should modify actual work or work because it copies the value immediately into the other field.