Can your query be written as:
select * from A where A.f1 NOT IN
(Select B.f1 from B join C where B.f2 = A.f2 and B.f3 = A.f3 and B.f4 = A.f4 and C.x = "VAL")
If yes, then you should use FieldCompareSetPredicate to implement the Not IN predicate.
Please refer to the LLBLGen Pro manual Using the generated code -> Adapter/SelfServicing -> Filtering and sorting -> The predicate system
If not, then please post a sample query replica based on Northwind so I can check it out.
Thanks.