So you have this relationship (but with different table and field names):
Parent::Field1 = Portal::Field2 AND
Parent::Field3 = Portal::field4
Do you have values in Parent::Field1 and Parent::field3?
If you enable "Allow creation of records via this relationship" for Portal, you don't have to enter values into Portal::field2 and Portal::Field4 at all. FileMaker will add those values for you from the matching fields in Parent when you enter data in any field from Portal using the bottom blank row of your portal.