If you check the FAQ, they bill you based on your peak hourly retrieval rate.
If I download 4TB at say...10MB/s, not only do I need to pay $480, but I also have to pay ~$257 as a retrieval fee?
Their wording is confusing, but ignoring the free retrieval amount (negligible difference on a 4TB transfer):
Fee = Peak hourly retrieval * number of hours in month * $0.01
https://aws.amazon.com/glacier/faqs/#How_much_data_can_I_ret...
Admittedly ~$737 isn't the end of the world if your house has burned down and you need all your data back, but it's still important to know the details.
I think in that situation, it would be cheaper to use their bulk import/export, which would be roughly $300 for 4TB