Azure Virtual WAN hub gateway capacity is configured using scale units. To meet the requirements while minimizing costs, you must select the lowest number of scale units that satisfy the specified throughput.

• S2S VPN Gateway: Each scale unit provides 500 Mbps of aggregate throughput. To support the required 4 Gbps (4000 Mbps), you need 4000 Mbps/500 Mbps per unit=8 scale units.
• ExpressRoute Gateway: Each scale unit provides 2 Gbps of aggregate throughput. To support the required 8 Gbps, you need 8 Gbps/2 Gbps per unit=4 scale units.

This configuration meets the exact performance needs without over-provisioning, thereby minimizing costs.

Microsoft Learn, "About Virtual WAN gateway settings." This document provides the official tables detailing the relationship between scale units and throughput for different gateway types.

Under the section "Gateway scale units," the "VPN scale units" table explicitly states that 1 scale unit equals 500 Mbps.

The "ExpressRoute scale units" table in the same section shows that 1 scale unit equals 2 Gbps.

Microsoft Azure, "Virtual WAN pricing." The official pricing page reinforces these values. In the pricing calculator and details for Virtual WAN, the "VPN Gateway Scale Unit" is defined at 500 Mbps increments, and the "ExpressRoute Gateway Scale Unit" is defined at 2 Gbps increments, directly linking cost to these performance metrics.