I love a good Relationship puzzle and I have created a test app which seems to work. Contact me via my profile and I will give you admin access.
https://yqc.quickbase.com/db/bj57f3zxg Basically I have a field called [Count 1] which always has a formula value of 1. One each of the three relationships I have a summary maximum of that field to determine of the product has children at that level. My summary field will have either a 1 or 0 in it as it's a summary maximum of the count 1 field which always has a value of 1.
Then my Hierarchy formula is
If(
[Product has Level B records (=1)]=1,4,
[Product has level C Records (=1)]=1,3,
[Product has Level D records (=1)]=1,2,1)
