Overbooking works differently depending on how close you are to departure time. If there are 4J seats remaining, then there will be 4 seats in J or above remaining on that flight. For example, if a flight has 20 seats available but is oversold in Economy by 10 at T-24hrs, then they will still sell Economy Class seats because it's easier to sell a $400 seat than a $4000 seat.

DiamondMile is correct in saying that staff (standby) tickets are waiting to be uplifted into those seats, or as a matter of fact: any seat. They can also be accepted into a lower cabin even though they've 'booked' a J seat, should they wish to.