Not sure if this is quite there,
but you could try adjusting the Day using:
IF DATEPART( 'hour', [Time Field] ) < 4
THEN DATEADD( 'day', -1, DATETRUNC( 'day', [Time Field] ) )
ELSE DATETRUNC( 'day', [Time Field] )
This shifts anything earlier than 4 to the previous day.
Please see workbook v10.3 attached in the Forum Thread:
285882day.twbx 54.9 KB