The total cost of Azure File Sync services is determined by the number of servers that connect to the cloud endpoint (Azure file share) plus the underlying costs of Azure Files (including storage and access costs) and outbound data transfer.
One sync server is free per storage sync service, any additional server costs $5
All Ingress traffic to Azure is free you only pay for egress traffic
So, in your case
4TB on prem to azure no cost
4TB of data on Azure Files Hot tier General purpose v2 Redundancy LRS = $173.34
or
4TB of data on Azure Files Premium Redundancy LRS = $776.00
Azure File Sync server 1 free+ additional 2<sup>nd</sup> sync server = $5
Egress 4TB traffic to second server on public internet = $312.00
you also need to be aware of the traffic from clients on premises requesting the files
ref:https://azure.microsoft.com/en-us/pricing/details/storage/files/
Cheers
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