Why does it take mail from Winston Salem, NC to Kernersville ( same city) three to four days to arrive in my mailbox?this happens about 90 percent of the time.

All if the mail goes through Greensboro first.

Mail that comes in on the day is separated on grade of importance (standard, first class, priority, etc) and it may take a day or two to be processed based on priority. Then be delivered the day its processed (normal) or the day after if there was a problem along the way. (Ranging from a miss-sort/ in the deliver tray to a machine breaking down somewhere)