Hi,
This rings a bell... if I remember right, you have to check that your FKKOP custom fields have been added to all the relevant structures.
For example, did you add the custom fields to structure FKKRW_FKKOP ?
I have used event 4000 in the past, and it definitely works for standard DFKKOP fields. Try debugging 4000 to see where the custom value is 'lost'.
cheers
Paul