The fourth time the unit is brought in for repair is the replacement event. It doesn't have to be for the same problem, though. If there are 4 totally separate repair events, (I.E. one trip for the LCD, one trip for the touchpad, one trip for the keyboard, one trip for the media reader), it's a lemon and gets replaced. If they fix 3 things at once, that is a single repair event. Some states have laws that supersede this, but I think anything like that is printed on your original warranty ticket.