Assuming that this database has been building up over the past decade, I'm in awe over how much money they must have spent on Laptop's in the early part of this decade.
It's probably as poster above says: each snapshot (sounds like it's DB + app instance or something, maybe a VM snapshot?) is probably > 1TB in size. If you have a 1.5TB image it would make sense to want 4TB storage so that you could have two versions locally at a time. Also explains how long it takes them to do things. Every time they switch versions they have to swap out TBs of data to pull latest. Because it also sounds like they don't have a way to grab just the diffs. They grab whole images at a time.