You could set them up to run a Restic server for you and then do backups with that. Restic is fully encrypted, does differential backups, and can do configurable pruning. With 4 TB the initial run will take long, even locally, but it can also resume, so should work.
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You could set them up to run a Restic server for you and then do backups with that. Restic is fully encrypted, does differential backups, and can do configurable pruning.