You need a relationship like this:
Then you can base your value list on values from PropertyTrees and specify one of the other three occurrences as your "starting from" table.
Thanks PhilModjunk for the quick reply. Although I'm not sure I understand correctly.
Is this (highlighted) the TO I need to create?
And the value list? This appears to be working correctly, but I just wanted an expert opinion.
That's what I had in mind, though I wasn't 100% sure if setting up a relaitonship "upstream" from the portal to ServiceLines would work until you tried it.