If your data is in S3, my experience is that you can push ~20-40MB/core/sec on most instances.

OP is probably talking about an x1e.32xlarge. According to Daniel Vassalo's S3 benchmark [1], it can do about 2.7GB/sec.

So your 4TB DB might take ~30min to fetch.

Bandwidth is free, you'd pay $2 for the 30 min of compute, and some fractions of pennies for the few hundred S3 requests.

[1]: https://github.com/dvassallo/s3-benchmark