Your math looks right. However, 4TB of data is way over the average amount of data most people have. The current online backup solutions are geared more towards the average user who has backup needs in the range of 10s of gigabytes.

Beyond that, some of the backup providers do clever tricks to minimize data transfer. One of them out there that does a complete system backup won't actually transfer files over the wire if they see a duplicate already stored in their system from another user. This cuts down on a lot of bulk data from Windows and installed applications, leaving just the users specific data going over the wire. Others just ignore all system/application files and only back up what a user has in My Documents.

One final trick I know of is that some backup providers allow you to mail them a drive for the initial backup, this ensuring only differences from that backup have to go over the wire.

For me, I only backup about 30 gigs offsite, with most of it sitting on a Time Capsule at my grandparents house two states over. The rest of my data I'm not overly concerned with losing. I do have local backups in the house to a RAID in a NAS, and if a large enough disaster strikes to wipe that out, I have bigger problems to deal with.