Hi Dilanke,
Please be aware that extraction enhancement takes place at the moment of extraction. I.e. you cannot influence anymore whether a particular field needs to be delta relevant or not. The delta extraction is only enhanced with the additional field.
The logic of DataSource 0FI_AP_4 will most likely not take into account any changes of BKPF fields. I do not see at the moment an easy way to fulfill your requirement.
Best regards,
Sander