Your remote volume of 4 TB is enormous. What made you decide on that value?
When restoring any file (even a tiny one), Duplicati would need to download at least one remote volume.
It also explains the problem you are seeing with compaction. The compact operation is trying to reduce remote files and reclaim wasted space by downloading some volumes that contain wasted space and repackaging them.
I leave all my backups with a 50 MB remote volume size.
Check out this for more info: Choosing Sizes in Duplicati - Duplicati 2 User's Manual