You can leave home base knowing that leg 4 might need an extension. The idea is to evaluate the state of the crew before departing on leg 4 to see whether the flight can be completed safely.
This subject was addressed exactly a year ago on these forums:
http://www.pprune.org/african-aviati...-required.html
There is no mention in the law that you cannot depart home base knowing that an extension might be required.