I've always been bad at math but isn't 4TB for $400 approximately $0.10/GB ?

As another commenter mentioned EBS is quite a bit better than just a SSD in a number of ways. You can resize it, it has published resiliency numbers, quick and easy snapshots, etc etc

Also worth noting that the newest generation (GP3) is 0.08/GB/Month although like all things Amazon there are other charges to take into account.