Oh no - these birds are determined to give you a hard time - aren't they! <img src="/images/graemlins/smile.gif" alt="" />
At 4 weeks of age, those babies need their parents for another 3-4 weeks, but if this egg is fertile, it will hatch in 18 days. Its possible that eggs laid now will be broken and may never hatch. Its also possible that the parents will begin pecking at the first clutch of babies to chase them out to allow the 2nd clutch to hatch. You may have to remove the older babies & handfeed them to weaning if this happens or they may be injured or killed.
Its not unusual for the parents to start another clutch when the babies are 6 weeks as the older babies will be almost on their own by the time the next clutch hatches.