Hi Sergio,
I suggest to simplify it: use the F4 BAdI for both "default values" and derivation based on Continent. In my opinion this approach is more transparent and easier to handle.
In the F4 BAdI you can check if any input for a Continent variable is supplied. If yes, then use it to derive the Countries belonging to the Continent. Else supply the "default" Countries.
Best regards,
Sander